public static <M> StreamOperatorSpec<M, M> createMergeOperatorSpec(String opId) {
    return new MergeOperatorSpec<>(opId);
  }
Creates a {@link StreamOperatorSpec} with a merger function.

@param opId  the unique ID of the operator
@param <M>  the type of input message
@return  the {@link StreamOperatorSpec} for the merge
samza-core/src/main/java/org/apache/samza/operators/spec/OperatorSpecs.java