code: func (s *fs) GetMetadata(dgst digest.Digest, key string) ([]byte, error) {
  	s.RLock()
  	defer s.RUnlock()
  
  	if _, err := s.get(dgst); err != nil {
  		return nil, err
  	}
  	bytes, err := ioutil.ReadFile(filepath.Join(s.metadataDir(dgst), key))
  	if err != nil {
  		return nil, errors.Wrap(err, "failed to read metadata")
  	}
  	return bytes, nil
  }
docs: // GetMetadata returns metadata for a given digest.