tracer: add Flush method

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

metadata/context.go

@@ -39,8 +39,6 @@ func FromOutgoingContext(ctx context.Context) (Metadata, bool) {
 
 // FromContext returns metadata from the given context
 // returned metadata shoud not be modified or race condition happens
-//
-// Deprecated: use FromIncomingContext or FromOutgoingContext
 func FromContext(ctx context.Context) (Metadata, bool) {
 	if ctx == nil {
 		return nil, false
@@ -53,8 +51,6 @@ func FromContext(ctx context.Context) (Metadata, bool) {
 }
 
 // NewContext creates a new context with the given metadata
-//
-// Deprecated: use NewIncomingContext or NewOutgoingContext
 func NewContext(ctx context.Context, md Metadata) context.Context {
 	if ctx == nil {
 		ctx = context.Background()

tracer/noop.go

@@ -4,6 +4,8 @@ import (
 	"context"
 )
 
+var _ Tracer = (*noopTracer)(nil)
+
 type noopTracer struct {
 	opts Options
 }
@@ -21,6 +23,10 @@ func (t *noopTracer) Start(ctx context.Context, name string, opts ...SpanOption)
 	return NewSpanContext(ctx, span), span
 }
 
+func (t *noopTracer) Flush(ctx context.Context) error {
+	return nil
+}
+
 func (t *noopTracer) Init(opts ...Option) error {
 	for _, o := range opts {
 		o(&t.opts)

tracer/tracer.go

@@ -16,6 +16,8 @@ type Tracer interface {
 	Init(...Option) error
 	// Start a trace
 	Start(ctx context.Context, name string, opts ...SpanOption) (context.Context, Span)
+	// Flush flushes spans
+	Flush(ctx context.Context) error
 }
 
 type Span interface {