void combineRawHostAccountStorageStatsMap(
      Map<Long, Map<Short, Map<Short, ContainerStorageStats>>> combinedHostAccountStorageStatsMap,
      Map<Long, Map<Short, Map<Short, ContainerStorageStats>>> hostAccountStorageStatsMap) {
    for (Map.Entry<Long, Map<Short, Map<Short, ContainerStorageStats>>> hostAccountStorageStatsMapEntry : hostAccountStorageStatsMap
        .entrySet()) {
      long partitionId = hostAccountStorageStatsMapEntry.getKey();
      if (!combinedHostAccountStorageStatsMap.containsKey(partitionId)) {
        combinedHostAccountStorageStatsMap.put(partitionId, hostAccountStorageStatsMapEntry.getValue());
      } else {
        mergeAccountStorageStatsMap(combinedHostAccountStorageStatsMap.get(partitionId),
            hostAccountStorageStatsMapEntry.getValue());
      }
    }
  }
Combine the second map to the first map. The rule is simple, if the value exists in the first map, then this method
sum the value with the existing value in the first map and replace the existing value. If the value doesn't exist
in the first map, this method adds the value to the first map.
@param combinedHostAccountStorageStatsMap The result map
@param hostAccountStorageStatsMap The map to combine to the result map
ambry-clustermap/src/main/java/com/github/ambry/clustermap/MySqlClusterAggregator.java