[v4] improve metadata documentation (#216)

* add usage docs for context types and metadata, improve comments

* changes after review

metadata/context.go

@@ -0,0 +1,294 @@
+package metadata
+
+import (
+	"context"
+	"fmt"
+	"strings"
+)
+
+// In the metadata package, context and metadata are treated as immutable.
+// Deep copies of metadata are made to keep things safe and correct.
+// If a user takes a map and changes it across threads, it's their responsibility.
+//
+// 1. Incoming Context
+//
+// This context is provided by an external system and populated by the server or broker of the micro framework.
+// It should not be modified. The idea is to extract all necessary data from it,
+// validate the data, and transfer it into the current context.
+// After that, only the current context should be used throughout the code.
+//
+// 2. Current Context
+//
+// This is the context used during the execution flow.
+// You can add any needed metadata to it and pass it through your code.
+//
+// 3. Outgoing Context
+//
+// This context is for sending data to external systems.
+// You can add what you need before sending it out.
+// But it’s usually better to build and prepare this context right before making the external call,
+// instead of changing it in many places.
+//
+// Execution Flow:
+//
+// [External System]
+//       ↓
+// [Incoming Context]
+//       ↓
+// [Extract & Validate Metadata from Incoming Context]
+//       ↓
+// [Prepare Current Context]
+//       ↓
+// [Enrich Current Context]
+//       ↓
+// [Business Logic]
+//       ↓
+// [Prepare Outgoing Context]
+//       ↓
+// [External System Call]
+
+type (
+	metadataCurrentKey  struct{}
+	metadataIncomingKey struct{}
+	metadataOutgoingKey struct{}
+
+	rawMetadata struct {
+		md    Metadata
+		added [][]string
+	}
+)
+
+// NewContext creates a new context with the provided Metadata attached.
+// The Metadata must not be modified after calling this function.
+func NewContext(ctx context.Context, md Metadata) context.Context {
+	return context.WithValue(ctx, metadataCurrentKey{}, rawMetadata{md: md})
+}
+
+// NewIncomingContext creates a new context with the provided incoming Metadata attached.
+// The Metadata must not be modified after calling this function.
+func NewIncomingContext(ctx context.Context, md Metadata) context.Context {
+	return context.WithValue(ctx, metadataIncomingKey{}, rawMetadata{md: md})
+}
+
+// NewOutgoingContext creates a new context with the provided outgoing Metadata attached.
+// The Metadata must not be modified after calling this function.
+func NewOutgoingContext(ctx context.Context, md Metadata) context.Context {
+	return context.WithValue(ctx, metadataOutgoingKey{}, rawMetadata{md: md})
+}
+
+// AppendContext returns a new context with the provided key-value pairs (kv)
+// merged with any existing metadata in the context. For a description of kv,
+// please refer to the Pairs documentation.
+func AppendContext(ctx context.Context, kv ...string) context.Context {
+	if len(kv)%2 == 1 {
+		panic(fmt.Sprintf("metadata: AppendContext got an odd number of input pairs for metadata: %d", len(kv)))
+	}
+	md, _ := ctx.Value(metadataCurrentKey{}).(rawMetadata)
+	added := make([][]string, len(md.added)+1)
+	copy(added, md.added)
+	kvCopy := make([]string, 0, len(kv))
+	for i := 0; i < len(kv); i += 2 {
+		kvCopy = append(kvCopy, strings.ToLower(kv[i]), kv[i+1])
+	}
+	added[len(added)-1] = kvCopy
+	return context.WithValue(ctx, metadataCurrentKey{}, rawMetadata{md: md.md, added: added})
+}
+
+// AppendOutgoingContext returns a new context with the provided key-value pairs (kv)
+// merged with any existing metadata in the context. For a description of kv,
+// please refer to the Pairs documentation.
+func AppendOutgoingContext(ctx context.Context, kv ...string) context.Context {
+	if len(kv)%2 == 1 {
+		panic(fmt.Sprintf("metadata: AppendOutgoingContext got an odd number of input pairs for metadata: %d", len(kv)))
+	}
+	md, _ := ctx.Value(metadataOutgoingKey{}).(rawMetadata)
+	added := make([][]string, len(md.added)+1)
+	copy(added, md.added)
+	kvCopy := make([]string, 0, len(kv))
+	for i := 0; i < len(kv); i += 2 {
+		kvCopy = append(kvCopy, strings.ToLower(kv[i]), kv[i+1])
+	}
+	added[len(added)-1] = kvCopy
+	return context.WithValue(ctx, metadataOutgoingKey{}, rawMetadata{md: md.md, added: added})
+}
+
+// FromContext retrieves a deep copy of the metadata from the context and returns it
+// with a boolean indicating if it was found.
+func FromContext(ctx context.Context) (Metadata, bool) {
+	raw, ok := ctx.Value(metadataCurrentKey{}).(rawMetadata)
+	if !ok {
+		return nil, false
+	}
+	metadataSize := len(raw.md)
+	for i := range raw.added {
+		metadataSize += len(raw.added[i]) / 2
+	}
+
+	out := make(Metadata, metadataSize)
+	for k, v := range raw.md {
+		out[k] = copyOf(v)
+	}
+	for _, added := range raw.added {
+		if len(added)%2 == 1 {
+			panic(fmt.Sprintf("metadata: FromContext got an odd number of input pairs for metadata: %d", len(added)))
+		}
+
+		for i := 0; i < len(added); i += 2 {
+			out[added[i]] = append(out[added[i]], added[i+1])
+		}
+	}
+	return out, true
+}
+
+// MustContext retrieves a deep copy of the metadata from the context and panics
+// if the metadata is not found.
+func MustContext(ctx context.Context) Metadata {
+	md, ok := FromContext(ctx)
+	if !ok {
+		panic("missing metadata")
+	}
+	return md
+}
+
+// FromIncomingContext retrieves a deep copy of the metadata from the context and returns it
+// with a boolean indicating if it was found.
+func FromIncomingContext(ctx context.Context) (Metadata, bool) {
+	raw, ok := ctx.Value(metadataIncomingKey{}).(rawMetadata)
+	if !ok {
+		return nil, false
+	}
+	metadataSize := len(raw.md)
+	for i := range raw.added {
+		metadataSize += len(raw.added[i]) / 2
+	}
+
+	out := make(Metadata, metadataSize)
+	for k, v := range raw.md {
+		out[k] = copyOf(v)
+	}
+	for _, added := range raw.added {
+		if len(added)%2 == 1 {
+			panic(fmt.Sprintf("metadata: FromIncomingContext got an odd number of input pairs for metadata: %d", len(added)))
+		}
+
+		for i := 0; i < len(added); i += 2 {
+			out[added[i]] = append(out[added[i]], added[i+1])
+		}
+	}
+	return out, true
+}
+
+// MustIncomingContext retrieves a deep copy of the metadata from the context and panics
+// if the metadata is not found.
+func MustIncomingContext(ctx context.Context) Metadata {
+	md, ok := FromIncomingContext(ctx)
+	if !ok {
+		panic("missing metadata")
+	}
+	return md
+}
+
+// FromOutgoingContext retrieves a deep copy of the metadata from the context and returns it
+// with a boolean indicating if it was found.
+func FromOutgoingContext(ctx context.Context) (Metadata, bool) {
+	raw, ok := ctx.Value(metadataOutgoingKey{}).(rawMetadata)
+	if !ok {
+		return nil, false
+	}
+
+	metadataSize := len(raw.md)
+	for i := range raw.added {
+		metadataSize += len(raw.added[i]) / 2
+	}
+
+	out := make(Metadata, metadataSize)
+	for k, v := range raw.md {
+		out[k] = copyOf(v)
+	}
+	for _, added := range raw.added {
+		if len(added)%2 == 1 {
+			panic(fmt.Sprintf("metadata: FromOutgoingContext got an odd number of input pairs for metadata: %d", len(added)))
+		}
+
+		for i := 0; i < len(added); i += 2 {
+			out[added[i]] = append(out[added[i]], added[i+1])
+		}
+	}
+	return out, ok
+}
+
+// MustOutgoingContext retrieves a deep copy of the metadata from the context and panics
+// if the metadata is not found.
+func MustOutgoingContext(ctx context.Context) Metadata {
+	md, ok := FromOutgoingContext(ctx)
+	if !ok {
+		panic("missing metadata")
+	}
+	return md
+}
+
+// ValueFromCurrentContext retrieves a deep copy of the metadata for the given key
+// from the context, performing a case-insensitive search if needed. Returns nil if not found.
+func ValueFromCurrentContext(ctx context.Context, key string) []string {
+	md, ok := ctx.Value(metadataCurrentKey{}).(rawMetadata)
+	if !ok {
+		return nil
+	}
+
+	if v, ok := md.md[key]; ok {
+		return copyOf(v)
+	}
+	for k, v := range md.md {
+		// Case-insensitive comparison: Metadata is a map, and there's no guarantee
+		// that the Metadata attached to the context is created using our helper
+		// functions.
+		if strings.EqualFold(k, key) {
+			return copyOf(v)
+		}
+	}
+	return nil
+}
+
+// ValueFromIncomingContext retrieves a deep copy of the metadata for the given key
+// from the context, performing a case-insensitive search if needed. Returns nil if not found.
+func ValueFromIncomingContext(ctx context.Context, key string) []string {
+	raw, ok := ctx.Value(metadataIncomingKey{}).(rawMetadata)
+	if !ok {
+		return nil
+	}
+
+	if v, ok := raw.md[key]; ok {
+		return copyOf(v)
+	}
+	for k, v := range raw.md {
+		// Case-insensitive comparison: Metadata is a map, and there's no guarantee
+		// that the Metadata attached to the context is created using our helper
+		// functions.
+		if strings.EqualFold(k, key) {
+			return copyOf(v)
+		}
+	}
+	return nil
+}
+
+// ValueFromOutgoingContext retrieves a deep copy of the metadata for the given key
+// from the context, performing a case-insensitive search if needed. Returns nil if not found.
+func ValueFromOutgoingContext(ctx context.Context, key string) []string {
+	md, ok := ctx.Value(metadataOutgoingKey{}).(rawMetadata)
+	if !ok {
+		return nil
+	}
+
+	if v, ok := md.md[key]; ok {
+		return copyOf(v)
+	}
+	for k, v := range md.md {
+		// Case-insensitive comparison: Metadata is a map, and there's no guarantee
+		// that the Metadata attached to the context is created using our helper
+		// functions.
+		if strings.EqualFold(k, key) {
+			return copyOf(v)
+		}
+	}
+	return nil
+}

metadata/headers.go

@@ -2,18 +2,18 @@
 package metadata
 
 var (
-	// HeaderTopic is the header name that contains topic name
+	// HeaderTopic is the header name that contains topic name.
 	HeaderTopic = "Micro-Topic"
-	// HeaderContentType specifies content type of message
+	// HeaderContentType specifies content type of message.
 	HeaderContentType = "Content-Type"
-	// HeaderEndpoint specifies endpoint in service
+	// HeaderEndpoint specifies endpoint in service.
 	HeaderEndpoint = "Micro-Endpoint"
-	// HeaderService specifies service
+	// HeaderService specifies service.
 	HeaderService = "Micro-Service"
-	// HeaderTimeout specifies timeout of operation
+	// HeaderTimeout specifies timeout of operation.
 	HeaderTimeout = "Micro-Timeout"
-	// HeaderAuthorization specifies Authorization header
+	// HeaderAuthorization specifies Authorization header.
 	HeaderAuthorization = "Authorization"
-	// HeaderXRequestID specifies request id
+	// HeaderXRequestID specifies request id.
 	HeaderXRequestID = "X-Request-Id"
 )

metadata/helpers.go

@@ -0,0 +1,7 @@
+package metadata
+
+func copyOf(v []string) []string {
+	vals := make([]string, len(v))
+	copy(vals, v)
+	return vals
+}

metadata/iterator.go

@@ -0,0 +1,37 @@
+package metadata
+
+import "sort"
+
+type Iterator struct {
+	md   Metadata
+	keys []string
+	cur  int
+	cnt  int
+}
+
+// Next advances the iterator to the next element.
+func (iter *Iterator) Next(k *string, v *[]string) bool {
+	if iter.cur+1 > iter.cnt {
+		return false
+	}
+
+	if k != nil && v != nil {
+		*k = iter.keys[iter.cur]
+		vv := iter.md[*k]
+		*v = make([]string, len(vv))
+		copy(*v, vv)
+		iter.cur++
+	}
+	return true
+}
+
+// Iterator returns an iterator for iterating over metadata in sorted order.
+func (md Metadata) Iterator() *Iterator {
+	iter := &Iterator{md: md, cnt: len(md)}
+	iter.keys = make([]string, 0, iter.cnt)
+	for k := range md {
+		iter.keys = append(iter.keys, k)
+	}
+	sort.Strings(iter.keys)
+	return iter
+}

metadata/metadata.go

@@ -1,21 +1,18 @@
 package metadata
 
 import (
-	"context"
 	"fmt"
 	"net/textproto"
-	"sort"
 	"strings"
 )
 
-// defaultMetadataSize used when need to init new Metadata
+// defaultMetadataSize is used when initializing new Metadata.
 var defaultMetadataSize = 2
 
-// Metadata is a mapping from metadata keys to values. Users should use the following
-// two convenience functions New and Pairs to generate Metadata.
+// Metadata maps keys to values. Use the New, NewWithMetadata and Pairs functions to create it.
 type Metadata map[string][]string
 
-// New creates an zero Metadata.
+// New creates a zero-value Metadata with the specified size.
 func New(l int) Metadata {
 	if l == 0 {
 		l = defaultMetadataSize
@@ -24,7 +21,7 @@ func New(l int) Metadata {
 	return md
 }
 
-// NewWithMetadata creates an Metadata from a given key-value map.
+// NewWithMetadata creates a Metadata from the provided key-value map.
 func NewWithMetadata(m map[string]string) Metadata {
 	md := make(Metadata, len(m))
 	for key, val := range m {
@@ -33,8 +30,7 @@ func NewWithMetadata(m map[string]string) Metadata {
 	return md
 }
 
-// Pairs returns an Metadata formed by the mapping of key, value ...
-// Pairs panics if len(kv) is odd.
+// Pairs returns a Metadata formed from the key-value mapping. It panics if the length of kv is odd.
 func Pairs(kv ...string) Metadata {
 	if len(kv)%2 == 1 {
 		panic(fmt.Sprintf("metadata: Pairs got the odd number of input pairs for metadata: %d", len(kv)))
@@ -46,12 +42,19 @@ func Pairs(kv ...string) Metadata {
 	return md
 }
 
-// Len returns the number of items in Metadata.
-func (md Metadata) Len() int {
-	return len(md)
+// Join combines multiple Metadatas into a single Metadata.
+// The order of values for each key is determined by the order in which the Metadatas are provided to Join.
+func Join(mds ...Metadata) Metadata {
+	out := Metadata{}
+	for _, md := range mds {
+		for k, v := range md {
+			out[k] = append(out[k], v...)
+		}
+	}
+	return out
 }
 
-// Copy returns a copy of Metadata.
+// Copy returns a deep copy of Metadata.
 func Copy(src Metadata) Metadata {
 	out := make(Metadata, len(src))
 	for k, v := range src {
@@ -60,7 +63,7 @@ func Copy(src Metadata) Metadata {
 	return out
 }
 
-// Copy returns a copy of Metadata.
+// Copy returns a deep copy of Metadata.
 func (md Metadata) Copy() Metadata {
 	out := make(Metadata, len(md))
 	for k, v := range md {
@@ -69,7 +72,19 @@ func (md Metadata) Copy() Metadata {
 	return out
 }
 
-// AsMap returns a copy of Metadata with map[string]string.
+// CopyTo performs a deep copy of Metadata to the out.
+func (md Metadata) CopyTo(out Metadata) {
+	for k, v := range md {
+		out[k] = copyOf(v)
+	}
+}
+
+// Len returns the number of items in Metadata.
+func (md Metadata) Len() int {
+	return len(md)
+}
+
+// AsMap returns a deep copy of Metadata as a map[string]string
 func (md Metadata) AsMap() map[string]string {
 	out := make(map[string]string, len(md))
 	for k, v := range md {
@@ -78,8 +93,7 @@ func (md Metadata) AsMap() map[string]string {
 	return out
 }
 
-// AsHTTP1 returns a copy of Metadata
-// with CanonicalMIMEHeaderKey.
+// AsHTTP1 returns a deep copy of Metadata with keys converted to canonical MIME header key format.
 func (md Metadata) AsHTTP1() map[string][]string {
 	out := make(map[string][]string, len(md))
 	for k, v := range md {
@@ -88,8 +102,7 @@ func (md Metadata) AsHTTP1() map[string][]string {
 	return out
 }
 
-// AsHTTP1 returns a copy of Metadata
-// with strings.ToLower.
+// AsHTTP2 returns a deep copy of Metadata with keys converted to lowercase.
 func (md Metadata) AsHTTP2() map[string][]string {
 	out := make(map[string][]string, len(md))
 	for k, v := range md {
@@ -98,14 +111,10 @@ func (md Metadata) AsHTTP2() map[string][]string {
 	return out
 }
 
-// CopyTo copies Metadata to out.
-func (md Metadata) CopyTo(out Metadata) {
-	for k, v := range md {
-		out[k] = copyOf(v)
-	}
-}
-
-// Get obtains the values for a given key.
+// Get retrieves the values for a given key, checking the key in three formats:
+// - exact case,
+// - lower case,
+// - canonical MIME header key format.
 func (md Metadata) Get(k string) []string {
 	v, ok := md[k]
 	if !ok {
@@ -117,13 +126,12 @@ func (md Metadata) Get(k string) []string {
 	return v
 }
 
-// GetJoined obtains the values for a given key
-// with joined values with "," symbol
+// GetJoined retrieves the values for a given key and joins them into a single string, separated by commas.
 func (md Metadata) GetJoined(k string) string {
 	return strings.Join(md.Get(k), ",")
 }
 
-// Set sets the value of a given key with a slice of values.
+// Set assigns the values to the given key.
 func (md Metadata) Set(key string, vals ...string) {
 	if len(vals) == 0 {
 		return
@@ -131,8 +139,7 @@ func (md Metadata) Set(key string, vals ...string) {
 	md[key] = vals
 }
 
-// Append adds the values to key k, not overwriting what was already stored at
-// that key.
+// Append adds values to the existing values for the given key.
 func (md Metadata) Append(key string, vals ...string) {
 	if len(vals) == 0 {
 		return
@@ -140,7 +147,10 @@ func (md Metadata) Append(key string, vals ...string) {
 	md[key] = append(md[key], vals...)
 }
 
-// Del removes the values for a given keys k.
+// Del removes the values for the given keys k. It checks and removes the keys in the following formats:
+// - exact case,
+// - lower case,
+// - canonical MIME header key format.
 func (md Metadata) Del(k ...string) {
 	for i := range k {
 		delete(md, k[i])
@@ -148,303 +158,3 @@ func (md Metadata) Del(k ...string) {
 		delete(md, textproto.CanonicalMIMEHeaderKey(k[i]))
 	}
 }
-
-// Join joins any number of Metadatas into a single Metadata.
-//
-// The order of values for each key is determined by the order in which the Metadatas
-// containing those values are presented to Join.
-func Join(mds ...Metadata) Metadata {
-	out := Metadata{}
-	for _, Metadata := range mds {
-		for k, v := range Metadata {
-			out[k] = append(out[k], v...)
-		}
-	}
-	return out
-}
-
-type (
-	metadataIncomingKey struct{}
-	metadataOutgoingKey struct{}
-	metadataCurrentKey  struct{}
-)
-
-// NewContext creates a new context with Metadata attached. Metadata must
-// not be modified after calling this function.
-func NewContext(ctx context.Context, md Metadata) context.Context {
-	return context.WithValue(ctx, metadataCurrentKey{}, rawMetadata{md: md})
-}
-
-// NewIncomingContext creates a new context with incoming Metadata attached. Metadata must
-// not be modified after calling this function.
-func NewIncomingContext(ctx context.Context, md Metadata) context.Context {
-	return context.WithValue(ctx, metadataIncomingKey{}, rawMetadata{md: md})
-}
-
-// NewOutgoingContext creates a new context with outgoing Metadata attached. If used
-// in conjunction with AppendOutgoingContext, NewOutgoingContext will
-// overwrite any previously-appended metadata. Metadata must not be modified after
-// calling this function.
-func NewOutgoingContext(ctx context.Context, md Metadata) context.Context {
-	return context.WithValue(ctx, metadataOutgoingKey{}, rawMetadata{md: md})
-}
-
-// AppendContext returns a new context with the provided kv merged
-// with any existing metadata in the context. Please refer to the documentation
-// of Pairs for a description of kv.
-func AppendContext(ctx context.Context, kv ...string) context.Context {
-	if len(kv)%2 == 1 {
-		panic(fmt.Sprintf("metadata: AppendContext got an odd number of input pairs for metadata: %d", len(kv)))
-	}
-	md, _ := ctx.Value(metadataCurrentKey{}).(rawMetadata)
-	added := make([][]string, len(md.added)+1)
-	copy(added, md.added)
-	kvCopy := make([]string, 0, len(kv))
-	for i := 0; i < len(kv); i += 2 {
-		kvCopy = append(kvCopy, strings.ToLower(kv[i]), kv[i+1])
-	}
-	added[len(added)-1] = kvCopy
-	return context.WithValue(ctx, metadataCurrentKey{}, rawMetadata{md: md.md, added: added})
-}
-
-// AppendOutgoingContext returns a new context with the provided kv merged
-// with any existing metadata in the context. Please refer to the documentation
-// of Pairs for a description of kv.
-func AppendOutgoingContext(ctx context.Context, kv ...string) context.Context {
-	if len(kv)%2 == 1 {
-		panic(fmt.Sprintf("metadata: AppendOutgoingContext got an odd number of input pairs for metadata: %d", len(kv)))
-	}
-	md, _ := ctx.Value(metadataOutgoingKey{}).(rawMetadata)
-	added := make([][]string, len(md.added)+1)
-	copy(added, md.added)
-	kvCopy := make([]string, 0, len(kv))
-	for i := 0; i < len(kv); i += 2 {
-		kvCopy = append(kvCopy, strings.ToLower(kv[i]), kv[i+1])
-	}
-	added[len(added)-1] = kvCopy
-	return context.WithValue(ctx, metadataOutgoingKey{}, rawMetadata{md: md.md, added: added})
-}
-
-// FromContext returns the metadata in ctx if it exists.
-func FromContext(ctx context.Context) (Metadata, bool) {
-	raw, ok := ctx.Value(metadataCurrentKey{}).(rawMetadata)
-	if !ok {
-		return nil, false
-	}
-	metadataSize := len(raw.md)
-	for i := range raw.added {
-		metadataSize += len(raw.added[i]) / 2
-	}
-
-	out := make(Metadata, metadataSize)
-	for k, v := range raw.md {
-		out[k] = copyOf(v)
-	}
-	for _, added := range raw.added {
-		if len(added)%2 == 1 {
-			panic(fmt.Sprintf("metadata: FromContext got an odd number of input pairs for metadata: %d", len(added)))
-		}
-
-		for i := 0; i < len(added); i += 2 {
-			out[added[i]] = append(out[added[i]], added[i+1])
-		}
-	}
-	return out, true
-}
-
-// MustContext returns the metadata in ctx.
-func MustContext(ctx context.Context) Metadata {
-	md, ok := FromContext(ctx)
-	if !ok {
-		panic("missing metadata")
-	}
-	return md
-}
-
-// FromIncomingContext returns the incoming metadata in ctx if it exists.
-func FromIncomingContext(ctx context.Context) (Metadata, bool) {
-	raw, ok := ctx.Value(metadataIncomingKey{}).(rawMetadata)
-	if !ok {
-		return nil, false
-	}
-	metadataSize := len(raw.md)
-	for i := range raw.added {
-		metadataSize += len(raw.added[i]) / 2
-	}
-
-	out := make(Metadata, metadataSize)
-	for k, v := range raw.md {
-		out[k] = copyOf(v)
-	}
-	for _, added := range raw.added {
-		if len(added)%2 == 1 {
-			panic(fmt.Sprintf("metadata: FromIncomingContext got an odd number of input pairs for metadata: %d", len(added)))
-		}
-
-		for i := 0; i < len(added); i += 2 {
-			out[added[i]] = append(out[added[i]], added[i+1])
-		}
-	}
-	return out, true
-}
-
-// MustIncomingContext returns the incoming metadata in ctx.
-func MustIncomingContext(ctx context.Context) Metadata {
-	md, ok := FromIncomingContext(ctx)
-	if !ok {
-		panic("missing metadata")
-	}
-	return md
-}
-
-// ValueFromIncomingContext returns the metadata value corresponding to the metadata
-// key from the incoming metadata if it exists. Keys are matched in a case insensitive
-// manner.
-func ValueFromIncomingContext(ctx context.Context, key string) []string {
-	raw, ok := ctx.Value(metadataIncomingKey{}).(rawMetadata)
-	if !ok {
-		return nil
-	}
-
-	if v, ok := raw.md[key]; ok {
-		return copyOf(v)
-	}
-	for k, v := range raw.md {
-		// Case insensitive comparison: Metadata is a map, and there's no guarantee
-		// that the Metadata attached to the context is created using our helper
-		// functions.
-		if strings.EqualFold(k, key) {
-			return copyOf(v)
-		}
-	}
-	return nil
-}
-
-// ValueFromCurrentContext returns the metadata value corresponding to the metadata
-// key from the incoming metadata if it exists. Keys are matched in a case insensitive
-// manner.
-func ValueFromCurrentContext(ctx context.Context, key string) []string {
-	md, ok := ctx.Value(metadataCurrentKey{}).(rawMetadata)
-	if !ok {
-		return nil
-	}
-
-	if v, ok := md.md[key]; ok {
-		return copyOf(v)
-	}
-	for k, v := range md.md {
-		// Case insensitive comparison: Metadata is a map, and there's no guarantee
-		// that the Metadata attached to the context is created using our helper
-		// functions.
-		if strings.EqualFold(k, key) {
-			return copyOf(v)
-		}
-	}
-	return nil
-}
-
-// MustOutgoingContext returns the outgoing metadata in ctx.
-func MustOutgoingContext(ctx context.Context) Metadata {
-	md, ok := FromOutgoingContext(ctx)
-	if !ok {
-		panic("missing metadata")
-	}
-	return md
-}
-
-// ValueFromOutgoingContext returns the metadata value corresponding to the metadata
-// key from the incoming metadata if it exists. Keys are matched in a case insensitive
-// manner.
-func ValueFromOutgoingContext(ctx context.Context, key string) []string {
-	md, ok := ctx.Value(metadataOutgoingKey{}).(rawMetadata)
-	if !ok {
-		return nil
-	}
-
-	if v, ok := md.md[key]; ok {
-		return copyOf(v)
-	}
-	for k, v := range md.md {
-		// Case insensitive comparison: Metadata is a map, and there's no guarantee
-		// that the Metadata attached to the context is created using our helper
-		// functions.
-		if strings.EqualFold(k, key) {
-			return copyOf(v)
-		}
-	}
-	return nil
-}
-
-func copyOf(v []string) []string {
-	vals := make([]string, len(v))
-	copy(vals, v)
-	return vals
-}
-
-// FromOutgoingContext returns the outgoing metadata in ctx if it exists.
-func FromOutgoingContext(ctx context.Context) (Metadata, bool) {
-	raw, ok := ctx.Value(metadataOutgoingKey{}).(rawMetadata)
-	if !ok {
-		return nil, false
-	}
-
-	metadataSize := len(raw.md)
-	for i := range raw.added {
-		metadataSize += len(raw.added[i]) / 2
-	}
-
-	out := make(Metadata, metadataSize)
-	for k, v := range raw.md {
-		out[k] = copyOf(v)
-	}
-	for _, added := range raw.added {
-		if len(added)%2 == 1 {
-			panic(fmt.Sprintf("metadata: FromOutgoingContext got an odd number of input pairs for metadata: %d", len(added)))
-		}
-
-		for i := 0; i < len(added); i += 2 {
-			out[added[i]] = append(out[added[i]], added[i+1])
-		}
-	}
-	return out, ok
-}
-
-type rawMetadata struct {
-	md    Metadata
-	added [][]string
-}
-
-// Iterator used to iterate over metadata with order
-type Iterator struct {
-	md   Metadata
-	keys []string
-	cur  int
-	cnt  int
-}
-
-// Next advance iterator to next element
-func (iter *Iterator) Next(k *string, v *[]string) bool {
-	if iter.cur+1 > iter.cnt {
-		return false
-	}
-
-	if k != nil && v != nil {
-		*k = iter.keys[iter.cur]
-		vv := iter.md[*k]
-		*v = make([]string, len(vv))
-		copy(*v, vv)
-		iter.cur++
-	}
-	return true
-}
-
-// Iterator returns the itarator for metadata in sorted order
-func (md Metadata) Iterator() *Iterator {
-	iter := &Iterator{md: md, cnt: len(md)}
-	iter.keys = make([]string, 0, iter.cnt)
-	for k := range md {
-		iter.keys = append(iter.keys, k)
-	}
-	sort.Strings(iter.keys)
-	return iter
-}