code: @Nonnull
    public ListenerRegistration addSnapshotListener(
        @Nonnull EventListener<DocumentSnapshot> listener) {
      return addSnapshotListener(firestore.getClient().getExecutor(), listener);
    }
docs: Starts listening to the document referenced by this DocumentReference.
  
  @param listener The event listener that will be called with the snapshots.
  @return A registration object that can be used to remove the listener.