code: func NewMemoryKeyCacher(maxKeyAge time.Duration, maxCacheSize int) KeyCacher {
  	return &memoryKeyCacher{
  		entries:      map[string]keyCacherEntry{},
  		maxKeyAge:    maxKeyAge,
  		maxCacheSize: maxCacheSize,
  	}
  }
docs: // NewMemoryKeyCacher creates a new Keycacher interface with option
  // to set max age of cached keys and max size of the cache.