code: func SeriesAndFieldFromCompositeKey(key []byte) ([]byte, []byte) {
  	sep := bytes.Index(key, keyFieldSeparatorBytes)
  	if sep == -1 {
  		// No field???
  		return key, nil
  	}
  	return key[:sep], key[sep+len(keyFieldSeparator):]
  }
docs: // SeriesAndFieldFromCompositeKey returns the series key and the field key extracted from the composite key.