code: func (mkc *memoryKeyCacher) keyIsExpired(keyID string) bool {
  	if time.Now().After(mkc.entries[keyID].addedAt.Add(mkc.maxKeyAge)) {
  		delete(mkc.entries, keyID)
  		return true
  	}
  	return false
  }
docs: // keyIsExpired deletes the key from cache if it is expired