code: public static <T> Maybe<Set<T>> keys(String key, Class<T> kClazz) {
          return keys(CacheService.CACHE_CONFIG_BEAN, key, kClazz);
      }
docs: retrial all the keys of the cached map
  
  @param key    cache key
  @param kClazz the key's class
  @return all the keys of the cached map, or null if the key does not exist