code: func mergeLabels(primary, secondary []prompb.Label) []prompb.Label {
  	result := make([]prompb.Label, 0, len(primary)+len(secondary))
  	i, j := 0, 0
  	for i < len(primary) && j < len(secondary) {
  		if primary[i].Name < secondary[j].Name {
  			result = append(result, primary[i])
  			i++
  		} else if primary[i].Name > secondary[j].Name {
  			result = append(result, secondary[j])
  			j++
  		} else {
  			result = append(result, primary[i])
  			i++
  			j++
  		}
  	}
  	for ; i < len(primary); i++ {
  		result = append(result, primary[i])
  	}
  	for ; j < len(secondary); j++ {
  		result = append(result, secondary[j])
  	}
  	return result
  }
docs: // mergeLabels merges two sets of sorted proto labels, preferring those in
  // primary to those in secondary when there is an overlap.