code: protected void emitWithOnlyKey(StreamMessage message, Object messageKey)
      {
          if (this.recordHistory)
          {
              message.getHeader().addHistory(messageKey.toString());
          }
  
          this.getCollector().emit(new Values("", message));
      }
docs: Use only MessageKey(Use key history's value) and not use MessageId(Id identify by storm).<br>
  Send message to downstream component.<br>
  Use following situation.
  <ol>
  <li>Use this class's key history function.</li>
  <li>Not use storm's fault detect function.</li>
  </ol>
  
  @param message sending message
  @param messageKey MessageKey(Use key history's value)