fix bug

http/context/context.go

@@ -21,6 +21,14 @@ func AppendToContext(ctx context.Context, key string, value interface{}) context
 		_value = fmt.Sprint(value)
 		break
 	}
-	ctx = metadata.AppendToOutgoingContext(ctx, key, _value)
+	md, ok := metadata.FromOutgoingContext(ctx)
+	if !ok {
+		md = make(map[string][]string)
+		md.Set(key, _value)
+		ctx = metadata.NewOutgoingContext(ctx, md)
+		return ctx
+	}
+	md.Set(key, _value)
+	ctx = metadata.NewOutgoingContext(ctx, md)
 	return ctx
 }