code: func AddUnsetLabelsToMap(aL map[string]string, labelsToAdd []string, labelSet labels.Set) {
  	for _, l := range labelsToAdd {
  		// if the label is already there, dont overwrite it.
  		if _, exists := aL[l]; exists {
  			continue
  		}
  		// otherwise, backfill this label.
  		if labelSet.Has(l) {
  			aL[l] = labelSet.Get(l)
  		}
  	}
  }
docs: // AddUnsetLabelsToMap backfills missing values with values we find in a map.