code: func (db *DB) StrLen(key []byte) (int64, error) {
  	s, err := db.GetSlice(key)
  	if err != nil {
  		return 0, err
  	}
  
  	n := s.Size()
  	s.Free()
  	return int64(n), nil
  }
docs: // StrLen returns the length of the data.