public static <T1, T2> void updateNestedMapHelper(Map<T1, Map<T2, Long>> nestedMap, T1 firstKey, T2 secondKey,
      Long value) {
    if (!nestedMap.containsKey(firstKey)) {
      nestedMap.put(firstKey, new HashMap<>());
    }
    updateMapHelper(nestedMap.get(firstKey), secondKey, value);
  }
Helper function to update nested map data structure.
@param nestedMap nested {@link Map} to be updated
@param firstKey of the nested map
@param secondKey of the nested map
@param value the value to be added at the corresponding entry
ambry-store/src/main/java/com/github/ambry/store/StatsUtils.java