code: func (h *headIndexReader) LabelValues(names ...string) (index.StringTuples, error) {
  	if len(names) != 1 {
  		return nil, encoding.ErrInvalidSize
  	}
  
  	h.head.symMtx.RLock()
  	sl := make([]string, 0, len(h.head.values[names[0]]))
  	for s := range h.head.values[names[0]] {
  		sl = append(sl, s)
  	}
  	h.head.symMtx.RUnlock()
  	sort.Strings(sl)
  
  	return index.NewStringTuples(sl, len(names))
  }
docs: // LabelValues returns the possible label values