原理：

将 ServiceDes很 implement 服务存储在 grpc.Server 的Map中！

用法：

  1.     // Register Greeter on the server.
  2.     hwpb.RegisterGreeterServer(s, &hwServer{})
  4.     // Register RouteGuide on the same server.
  5.     ecpb.RegisterEchoServer(s, &ecServer{})

调用方法：

  1.     fmt.Println("--- calling helloworld.Greeter/SayHello ---")
  2.     // Make a greeter client and send an RPC.
  3.     hwc := hwpb.NewGreeterClient(conn)
  4.     callSayHello(hwc, "multiplex")
  6.     fmt.Println()
  7.     fmt.Println("--- calling routeguide.RouteGuide/GetFeature ---")
  8.     // Make a routeguild client with the same ClientConn.
  9.     rgc := ecpb.NewEchoClient(conn)
  10.     callUnaryEcho(rgc, "this is examples/multiplex")