code: public RequestLimitRule matchingKeys(Set<String> keys) {
          return new RequestLimitRule(this.durationSeconds, this.limit, this.precision, this.name, keys);
      }
docs: Applies a key to the rate limit that defines to which keys, the rule applies, null for any unmatched key.
  
  @param keys Defines a set of keys to which the rule applies.
  @return a limit rule