code: public MessageStatListResult getMessageStatistic(String timeUnit, String start, int duration)
              throws APIConnectionException, APIRequestException {
          return _reportClient.getMessageStatistic(timeUnit, start, duration);
      }
docs: Get message statistic. Detail instructions please refer to https://docs.jiguang.cn/jmessage/server/rest_api_im_report_v2/#_6
  @param timeUnit MUST be one of HOUR, DAY, MONTH
  @param start start time, when timeUnit is HOUR, format: yyyy-MM-dd HH,
  @param duration depends on timeUnit, the duration has limit
  @return {@link MessageStatListResult}
  @throws APIConnectionException connect exception
  @throws APIRequestException request exception