code: func numLabelsToString(numLabels map[string][]int64, numUnits map[string][]string) string {
  	ls := []string{}
  	for k, v := range numLabels {
  		units := numUnits[k]
  		var labelString string
  		if len(units) == len(v) {
  			values := make([]string, len(v))
  			for i, vv := range v {
  				values[i] = fmt.Sprintf("%d %s", vv, units[i])
  			}
  			labelString = fmt.Sprintf("%s:%v", k, values)
  		} else {
  			labelString = fmt.Sprintf("%s:%v", k, v)
  		}
  		ls = append(ls, labelString)
  	}
  	sort.Strings(ls)
  	return strings.Join(ls, " ")
  }
docs: // numLabelsToString returns a string representation of a map
  // representing numeric labels.