docs: public ApiFuture<Void> add(final RowMutation rowMutation) {
      Preconditions.checkState(!closed, "BulkMutations has been closed");
      numOutstanding.incrementAndGet();
  
      ApiFuture<Void> future = callable.futureCall(rowMutation);
  
      ApiFutures.addCallback(
          future,
          new ApiFutureCallback<Void>() {
            @Override
            public void onFailure(Throwable throwable) {
              onMutationComplete(false);
            }
  
            @Override
            public void onSuccess(Void aVoid) {
              onMutationComplete(true);
            }
          },
          MoreExecutors.directExecutor());
  
      return future;
    }