[receiver/prometheusreceiver] Use Prometheus Labels public method API (…

…#31908)

**Description:**
By only using the public method API for Prometheus labels (rather than
assuming `labels.Labels` is an alias of a slice) it opens up the
possibility to build a collector with the `stringlabels` tag, so we can
use the more memory efficient labels implementation.

**Link to tracking Issue:** #31907

**Testing:** 
I had trouble running all of the tests locally, so I think I will need
some help with making that work. I did run all the tests I changed with
`-tags=stringlabels` and without it.

receiver/prometheusreceiver/internal/metricfamily.go

@@ -240,19 +240,19 @@ func populateAttributes(mType pmetric.MetricType, ls labels.Labels, dest pcommon
 	dest.EnsureCapacity(ls.Len())
 	names := getSortedNotUsefulLabels(mType)
 	j := 0
-	for i := range ls {
-		for j < len(names) && names[j] < ls[i].Name {
+	ls.Range(func(l labels.Label) {
+		for j < len(names) && names[j] < l.Name {
 			j++
 		}
-		if j < len(names) && ls[i].Name == names[j] {
-			continue
+		if j < len(names) && l.Name == names[j] {
+			return
 		}
-		if ls[i].Value == "" {
+		if l.Value == "" {
 			// empty label values should be omitted
-			continue
+			return
 		}
-		dest.PutStr(ls[i].Name, ls[i].Value)
-	}
+		dest.PutStr(l.Name, l.Value)
+	})
 }
 
 func (mf *metricFamily) loadMetricGroupOrCreate(groupKey uint64, ls labels.Labels, ts int64) *metricGroup {
@@ -382,8 +382,8 @@ func (mf *metricFamily) addExemplar(seriesRef uint64, e exemplar.Exemplar) {
 func convertExemplar(pe exemplar.Exemplar, e pmetric.Exemplar) {
 	e.SetTimestamp(timestampFromMs(pe.Ts))
 	e.SetDoubleValue(pe.Value)
-	e.FilteredAttributes().EnsureCapacity(len(pe.Labels))
-	for _, lb := range pe.Labels {
+	e.FilteredAttributes().EnsureCapacity(pe.Labels.Len())
+	pe.Labels.Range(func(lb labels.Label) {
 		switch strings.ToLower(lb.Name) {
 		case traceIDKey:
 			var tid [16]byte
@@ -404,7 +404,7 @@ func convertExemplar(pe exemplar.Exemplar, e pmetric.Exemplar) {
 		default:
 			e.FilteredAttributes().PutStr(lb.Name, lb.Value)
 		}
-	}
+	})
 }
 
 /*

receiver/prometheusreceiver/internal/transaction.go

@@ -7,7 +7,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"sort"
 
 	"github.com/prometheus/common/model"
 	"github.com/prometheus/prometheus/model/exemplar"
@@ -91,9 +90,12 @@ func (t *transaction) Append(_ storage.SeriesRef, ls labels.Labels, atMs int64,
 	default:
 	}
 
-	if len(t.externalLabels) != 0 {
-		ls = append(ls, t.externalLabels...)
-		sort.Sort(ls)
+	if t.externalLabels.Len() != 0 {
+		b := labels.NewBuilder(ls)
+		t.externalLabels.Range(func(l labels.Label) {
+			b.Set(l.Name, l.Value)
+		})
+		ls = b.Labels()
 	}
 
 	if t.isNew {
@@ -259,14 +261,14 @@ func (t *transaction) getMetrics(resource pcommon.Resource) (pmetric.Metrics, er
 
 func getScopeID(ls labels.Labels) scopeID {
 	var scope scopeID
-	for _, lbl := range ls {
+	ls.Range(func(lbl labels.Label) {
 		if lbl.Name == scopeNameLabel {
 			scope.name = lbl.Value
 		}
 		if lbl.Name == scopeVersionLabel {
 			scope.version = lbl.Value
 		}
-	}
+	})
 	return scope
 }
 
@@ -325,33 +327,33 @@ func (t *transaction) UpdateMetadata(_ storage.SeriesRef, _ labels.Labels, _ met
 	return 0, nil
 }
 
-func (t *transaction) AddTargetInfo(labels labels.Labels) {
+func (t *transaction) AddTargetInfo(ls labels.Labels) {
 	attrs := t.nodeResource.Attributes()
-	for _, lbl := range labels {
+	ls.Range(func(lbl labels.Label) {
 		if lbl.Name == model.JobLabel || lbl.Name == model.InstanceLabel || lbl.Name == model.MetricNameLabel {
-			continue
+			return
 		}
 		attrs.PutStr(lbl.Name, lbl.Value)
-	}
+	})
 }
 
-func (t *transaction) addScopeInfo(labels labels.Labels) {
+func (t *transaction) addScopeInfo(ls labels.Labels) {
 	attrs := pcommon.NewMap()
 	scope := scopeID{}
-	for _, lbl := range labels {
+	ls.Range(func(lbl labels.Label) {
 		if lbl.Name == model.JobLabel || lbl.Name == model.InstanceLabel || lbl.Name == model.MetricNameLabel {
-			continue
+			return
 		}
 		if lbl.Name == scopeNameLabel {
 			scope.name = lbl.Value
-			continue
+			return
 		}
 		if lbl.Name == scopeVersionLabel {
 			scope.version = lbl.Value
-			continue
+			return
 		}
 		attrs.PutStr(lbl.Name, lbl.Value)
-	}
+	})
 	t.scopeAttributes[scope] = attrs
 }