public static <M, OM> AsyncFlatMapOperatorSpec<M, OM> createAsyncOperatorSpec(
      AsyncFlatMapFunction<? super M, ? extends OM> asyncFlatMapFn, String opId) {
    return new AsyncFlatMapOperatorSpec<>((AsyncFlatMapFunction<M, OM>) asyncFlatMapFn, opId);
  }
Creates a {@link AsyncFlatMapOperatorSpec} for {@link AsyncFlatMapFunction}.

@param asyncFlatMapFn  the transformation function
@param opId  the unique ID of the operator
@param <M>  type of input message
@param <OM>  type of output message
@return  the {@link AsyncFlatMapOperatorSpec}
samza-core/src/main/java/org/apache/samza/operators/spec/OperatorSpecs.java