code: public static String getMetricNameWithUserTag(String metricName, String userName) {
      UserMetricKey k = new UserMetricKey(metricName, userName);
      String result = CACHED_METRICS.get(k);
      if (result != null) {
        return result;
      }
      return CACHED_METRICS.computeIfAbsent(k, key -> metricName + "." + CommonMetrics.TAG_USER
          + TAG_SEPARATOR + userName);
    }
docs: Gets a metric name with a specific user tag.
  
  @param metricName the name of the metric
  @param userName the user
  @return a metric name with the user tagged