message: add comment for simple_client
diff: @@ -90,6 +90,8 @@ int main(int argc, char* argv[]) {
  << "test_type = " << test_type
  << "sample_type = " << sample_type;
  std::unique_ptr<ServingClient> client;
  + // default type is brpc
  + // will add grpc&http in the future
  if (test_type == "brpc") {
  client.reset(new ServingBrpcClient());
  } else {
  