code: func (db *DB) ZCount(key []byte, min int64, max int64) (int64, error) {
  	if err := checkKeySize(key); err != nil {
  		return 0, err
  	}
  	minKey := db.zEncodeStartScoreKey(key, min)
  	maxKey := db.zEncodeStopScoreKey(key, max)
  
  	rangeType := store.RangeROpen
  
  	it := db.bucket.RangeLimitIterator(minKey, maxKey, rangeType, 0, -1)
  	var n int64
  	for ; it.Valid(); it.Next() {
  		n++
  	}
  	it.Close()
  
  	return n, nil
  }
docs: // ZCount gets the number of score in [min, max]