message: Add GoogleDefaultCredentials support
diff: @@ -193,8 +193,16 @@ class PubSubDatasetOp : public DatasetOpKernel {
  // Actually move on to next subscription.
  string subscription = dataset()->subscriptions_[current_subscription_index_];
  - // auto creds = grpc::GoogleDefaultCredentials();
  - stub_ = Subscriber::NewStub(grpc::CreateChannel(dataset()->server_, grpc::InsecureChannelCredentials()));
  + string server = dataset()->server_;
  + auto creds = grpc::GoogleDefaultCredentials();
  + if (dataset()->server_.find("http://") == 0) {
  + server = dataset()->server_.substr(7);
  + creds = grpc::InsecureChannelCredentials();
  + } else if (dataset()->server_.find("https://") == 0) {
  + // https://pubsub.googleapis.com
  + server = dataset()->server_.substr(8);
  + }
  + stub_ = Subscriber::NewStub(grpc::CreateChannel(server, creds));
  return Status::OK();
  }
  