public void addMultiStatistic(String statKey, String groupName, Statistic statistic) {
        addStatistic(statKey, statistic);
        List<String> group = multiStatGroups.get(groupName);
        if(group == null) {
            group = new ArrayList<>();
            multiStatGroups.put(groupName, group);
        }
        group.add(statKey);
        keyToGroupMap.put(statKey, groupName);
    }
Returns a statistic being tracked by the StatManager.

@param statKey The key of the definition.
@return Returns the related stat.
xmppserver/src/main/java/org/jivesoftware/openfire/stats/StatisticsManager.java