use metadata.Metadata (#8)

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

client/client.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/unistack-org/micro/v3/codec"
+	"github.com/unistack-org/micro/v3/metadata"
 )
 
 var (
@@ -57,7 +58,7 @@ type Response interface {
 	// Read the response
 	Codec() codec.Reader
 	// read the header
-	Header() map[string]string
+	Header() metadata.Metadata
 	// Read the undecoded response
 	Read() ([]byte, error)
 }

client/noop.go

@@ -66,14 +66,14 @@ func (n *noopRequest) Stream() bool {
 
 type noopResponse struct {
 	codec  codec.Reader
-	header map[string]string
+	header metadata.Metadata
 }
 
 func (n *noopResponse) Codec() codec.Reader {
 	return n.codec
 }
 
-func (n *noopResponse) Header() map[string]string {
+func (n *noopResponse) Header() metadata.Metadata {
 	return n.header
 }