code: @Nonnull
    public ListenerRegistration addSnapshotListener(
        @Nonnull Executor executor, @Nonnull EventListener<QuerySnapshot> listener) {
      return Watch.forQuery(this).runWatch(executor, listener);
    }
docs: Starts listening to this query.
  
  @param executor The executor to use to call the listener.
  @param listener The event listener that will be called with the snapshots.
  @return A registration object that can be used to remove the listener.