@VisibleForTesting
  public OperatorSpec<?, M> getOperatorSpec() {
    return this.operatorSpec;
  }
Get the {@link OperatorSpec} associated with this {@link MessageStreamImpl}.
@return the {@link OperatorSpec} associated with this {@link MessageStreamImpl}.
samza-core/src/main/java/org/apache/samza/operators/MessageStreamImpl.java