Add extensiong for metadata service

common/extension/service_name_mapping.go

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package extension
+
+import (
+	"github.com/apache/dubbo-go/metadata"
+)
+
+var(
+	nameMappings = make(map[string]func()metadata.ServiceNameMapping)
+)
+
+func SetServiceNameMapping(name string, creator func()metadata.ServiceNameMapping) {
+	// TODO(@邓大明)
+}
+
+func GetServiceNameMapping(name string) metadata.ServiceNameMapping {
+	// TODO(@邓大明)
+	return nil
+}