@VisibleForTesting
  final void onMessage(M message, MessageCollector collector, TaskCoordinator coordinator) {
    onMessageAsync(message, collector, coordinator)
        .toCompletableFuture().join();
  }
Get the unique ID for this {@link OperatorImpl} in the DAG.

Some {@link OperatorImpl}s don't have a 1:1 mapping with their {@link OperatorSpec}. E.g., there are
2 PartialJoinOperatorImpls for a JoinOperatorSpec. Overriding this method allows them to provide an
implementation specific id, e.g., for use in metrics.

@return the unique ID for this {@link OperatorImpl} in the DAG
samza-core/src/main/java/org/apache/samza/operators/impl/OperatorImpl.java