code: func (s *Store) Metadata(id, key string) string {
  	s.metaMu.RLock()
  	defer s.metaMu.RUnlock()
  
  	if _, ok := s.meta[id]; !ok {
  		return ""
  	}
  	v, ok := s.meta[id][key]
  	if ok {
  		return v
  	}
  	return ""
  }
docs: // Metadata returns the value for a given key, for a given node ID.