code: function ListenRequest(properties) {
                      this.labels = {};
                      if (properties)
                          for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                              if (properties[keys[i]] != null)
                                  this[keys[i]] = properties[keys[i]];
                  }
docs: Properties of a ListenRequest.
  @memberof google.firestore.v1
  @interface IListenRequest
  @property {string|null} [database] ListenRequest database
  @property {google.firestore.v1.ITarget|null} [addTarget] ListenRequest addTarget
  @property {number|null} [removeTarget] ListenRequest removeTarget
  @property {Object.<string,string>|null} [labels] ListenRequest labels
   
  Constructs a new ListenRequest.
  @memberof google.firestore.v1
  @classdesc Represents a ListenRequest.
  @implements IListenRequest
  @constructor
  @param {google.firestore.v1.IListenRequest=} [properties] Properties to set