code: public static String get(FullHttpRequest fullHttpRequest) {
          HttpHeaders httpHeaders = fullHttpRequest.headers();
          if (isNodeLegal(httpHeaders) && !StringUtil.isEmpty(httpHeaders.get(Constant.XIAN_MSG_ID_HEADER))) {
              return httpHeaders.get(Constant.XIAN_MSG_ID_HEADER);
          }
          return null;
      }
docs: 从httpRequest的Header内摘取出msgId，处于安全考虑，我们只摘取standalone节点传入的msgId <br>
  如果取不到那么返回null