code: public RequestLimitRule matchingKeys(String... keys) {
          Set<String> keySet = keys.length > 0 ? new HashSet<>(Arrays.asList(keys)) : null;
          return matchingKeys(keySet);
      }
docs: Applies a key to the rate limit that defines to which keys, the rule applies, empty for any unmatched key.
  
  @param keys Defines a set of keys to which the rule applies.
  @return a limit rule