Appending to metadata is a side effect. Let's not do that anymore.

metadata/metadata.go

@@ -17,12 +17,5 @@ func FromContext(ctx context.Context) (Metadata, bool) {
 }
 
 func NewContext(ctx context.Context, md Metadata) context.Context {
-	if emd, ok := ctx.Value(metaKey{}).(Metadata); ok {
-		for k, v := range emd {
-			if _, ok := md[k]; !ok {
-				md[k] = v
-			}
-		}
-	}
 	return context.WithValue(ctx, metaKey{}, md)
 }

wrapper.go

@@ -12,17 +12,30 @@ type clientWrapper struct {
 	headers metadata.Metadata
 }
 
+func (c *clientWrapper) setHeaders(ctx context.Context) context.Context {
+	md, ok := metadata.FromContext(ctx)
+	if !ok {
+		md = metadata.Metadata{}
+	}
+	for k, v := range c.headers {
+		if _, ok := md[k]; !ok {
+			md[k] = v
+		}
+	}
+	return metadata.NewContext(ctx, md)
+}
+
 func (c *clientWrapper) Call(ctx context.Context, req client.Request, rsp interface{}, opts ...client.CallOption) error {
-	ctx = metadata.NewContext(ctx, c.headers)
+	ctx = c.setHeaders(ctx)
 	return c.Client.Call(ctx, req, rsp, opts...)
 }
 
 func (c *clientWrapper) Stream(ctx context.Context, req client.Request, opts ...client.CallOption) (client.Streamer, error) {
-	ctx = metadata.NewContext(ctx, c.headers)
+	ctx = c.setHeaders(ctx)
 	return c.Client.Stream(ctx, req, opts...)
 }
 
 func (c *clientWrapper) Publish(ctx context.Context, p client.Publication, opts ...client.PublishOption) error {
-	ctx = metadata.NewContext(ctx, c.headers)
+	ctx = c.setHeaders(ctx)
 	return c.Client.Publish(ctx, p, opts...)
 }

wrapper_test.go

@@ -0,0 +1,55 @@
+package micro
+
+import (
+	"testing"
+
+	"github.com/micro/go-micro/metadata"
+
+	"golang.org/x/net/context"
+)
+
+func TestWrapper(t *testing.T) {
+	testData := []struct {
+		existing  metadata.Metadata
+		headers   metadata.Metadata
+		overwrite bool
+	}{
+		{
+			existing: metadata.Metadata{},
+			headers: metadata.Metadata{
+				"foo": "bar",
+			},
+			overwrite: true,
+		},
+		{
+			existing: metadata.Metadata{
+				"foo": "bar",
+			},
+			headers: metadata.Metadata{
+				"foo": "baz",
+			},
+			overwrite: false,
+		},
+	}
+
+	for _, d := range testData {
+		c := &clientWrapper{
+			headers: d.headers,
+		}
+
+		ctx := metadata.NewContext(context.Background(), d.existing)
+		c.setHeaders(ctx)
+
+		md, _ := metadata.FromContext(ctx)
+
+		for k, v := range d.headers {
+			if d.overwrite && md[k] != v {
+				t.Fatalf("Expected %s=%s got %s=%s", k, v, k, md[k])
+			}
+			if !d.overwrite && md[k] != d.existing[k] {
+				t.Fatalf("Expected %s=%s got %s=%s", k, d.existing[k], k, md[k])
+			}
+		}
+	}
+
+}