code: function _getUnaryListener(method_definition, emitter, callback) {
    var resultMessage;
    return {
      onReceiveMetadata: function (metadata) {
        emitter.emit('metadata', metadata);
      },
      onReceiveMessage: function (message) {
        resultMessage = message;
      },
      onReceiveStatus: function (status) {
        if (status.code !== constants.status.OK) {
          var error = common.createStatusError(status);
          callback(error);
        } else {
          callback(null, resultMessage);
        }
        emitter.emit('status', status);
      }
    };
  }
docs: Produces a listener for responding to callers of unary RPCs.
  @private
  @param {grpc~MethodDefinition} method_definition
  @param {EventEmitter} emitter
  @param {function} callback
  @return {grpc~Listener}