use metadata.Metadata (#8)

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

util/stream/stream.go

@@ -35,7 +35,7 @@ func (r *request) Codec() codec.Reader {
 	return r.Request.Codec().(codec.Reader)
 }
 
-func (r *request) Header() map[string]string {
+func (r *request) Header() metadata.Metadata {
 	md, _ := metadata.FromContext(r.context)
 	return md
 }

util/token/jwt/jwt.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/dgrijalva/jwt-go"
 	"github.com/unistack-org/micro/v3/auth"
+	"github.com/unistack-org/micro/v3/metadata"
 	"github.com/unistack-org/micro/v3/util/token"
 )
 
@@ -13,7 +14,7 @@ import (
 type authClaims struct {
 	Type     string            `json:"type"`
 	Scopes   []string          `json:"scopes"`
-	Metadata map[string]string `json:"metadata"`
+	Metadata metadata.Metadata `json:"metadata"`
 
 	jwt.StandardClaims
 }