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)
 }