code: @Nonnull
    public ApiFuture<List<DocumentSnapshot>> getAll(
        @Nonnull DocumentReference[] documentReferences, @Nullable FieldMask fieldMask) {
      Preconditions.checkState(isEmpty(), READ_BEFORE_WRITE_ERROR_MSG);
  
      return firestore.getAll(documentReferences, fieldMask, transactionId);
    }
docs: Retrieves multiple documents from Firestore, while optionally applying a field mask to reduce
  the amount of data transmitted from the backend. Holds a pessimistic lock on all returned
  documents.
  
  @param documentReferences Array with Document References to fetch.
  @param fieldMask If set, specifies the subset of fields to return.