code: func MustEnsureIndexExpireAt() Option {
  	return func(m *MongoCache) {
  		if err := m.EnsureIndex(); err != nil {
  			panic(fmt.Sprintf("index must ensure %q", err))
  		}
  	}
  }
docs: // MustEnsureIndexExpireAt ensures the expireAt index
  // usage:
  // NewMongoCacheWithTTL(mongoSession, MustEnsureIndexExpireAt())