code: function sendMetadata(responseMetadata) {
    /* jshint validthis: true */
    var self = this;
    if (!this.call.metadataSent) {
      this.call.metadataSent = true;
      var batch = {};
      batch[grpc.opType.SEND_INITIAL_METADATA] =
          responseMetadata._getCoreRepresentation();
      this.call.startBatch(batch, function(err) {
        if (err) {
          self.emit('error', err);
          return;
        }
      });
    }
  }
docs: Send the initial metadata for a writable stream.
  @alias grpc~ServerUnaryCall#sendMetadata
  @param {grpc.Metadata} responseMetadata Metadata to send