Fix errors from test scripts.

include/grpcpp/ext/channelz_service_plugin.h

@@ -21,4 +21,16 @@
 
 #include <grpcpp/ext/channelz_service_plugin_impl.h>
 
+namespace grpc {
+namespace channelz {
+namespace experimental {
+
+static inline void InitChannelzService() {
+  ::grpc_impl::channelz::experimental::InitChannelzService();
+}
+
+}  // namespace experimental
+}  // namespace channelz
+}  // namespace grpc
+
 #endif  // GRPCPP_EXT_CHANNELZ_SERVICE_PLUGIN_H

src/cpp/server/channelz/channelz_service_plugin.cc

@@ -67,13 +67,21 @@ CreateChannelzServicePlugin() {
       new ChannelzServicePlugin());
 }
 
+}  // namespace experimental
+}  // namespace channelz
+}  // namespace grpc
+namespace grpc_impl {
+namespace channelz {
+namespace experimental {
+
 void InitChannelzService() {
   static bool already_here = false;
   if (already_here) return;
   already_here = true;
-  ::grpc::ServerBuilder::InternalAddPluginFactory(&CreateChannelzServicePlugin);
+  ::grpc::ServerBuilder::InternalAddPluginFactory(
+      &grpc::channelz::experimental::CreateChannelzServicePlugin);
 }
 
 }  // namespace experimental
 }  // namespace channelz
-}  // namespace grpc
+}  // namespace grpc_impl