code: func (db *DB) ZCard(key []byte) (int64, error) {
  	if err := checkKeySize(key); err != nil {
  		return 0, err
  	}
  
  	sk := db.zEncodeSizeKey(key)
  	return Int64(db.bucket.Get(sk))
  }
docs: // ZCard gets the size of the zset.