code: func (h *headIndexReader) LabelNames() ([]string, error) {
  	h.head.symMtx.RLock()
  	defer h.head.symMtx.RUnlock()
  	labelNames := make([]string, 0, len(h.head.values))
  	for name := range h.head.values {
  		if name == "" {
  			continue
  		}
  		labelNames = append(labelNames, name)
  	}
  	sort.Strings(labelNames)
  	return labelNames, nil
  }
docs: // LabelNames returns all the unique label names present in the head.