@Override
  protected CompletionStage<Collection<RM>> handleMessageAsync(M message, MessageCollector collector,
      TaskCoordinator coordinator) {
    return CompletableFuture.completedFuture(this.transformFn.apply(message));
  }
A simple operator that accepts a 1:n transform function and applies it to each incoming message.

@param <M>  the type of input message
@param <RM>  the type of result
samza-core/src/main/java/org/apache/samza/operators/impl/FlatmapOperatorImpl.java