store: compare labels directly (thanos-io#7766)

Do not create intermediate prometheus labels and compare the labels
directly.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

pkg/store/proxy_merge.go

@@ -173,10 +173,7 @@ func NewProxyResponseLoserTree(seriesSets ...respSet) *losertree.Tree[*storepb.S
 			return true
 		}
 		if a.GetSeries() != nil && b.GetSeries() != nil {
-			iLbls := labelpb.LabelpbLabelsToPromLabels(a.GetSeries().Labels)
-			jLbls := labelpb.LabelpbLabelsToPromLabels(b.GetSeries().Labels)
-
-			return labels.Compare(iLbls, jLbls) < 0
+			return labelpb.CompareLabels(a.GetSeries().Labels, b.GetSeries().Labels) < 0
 		} else if a.GetSeries() == nil && b.GetSeries() != nil {
 			return true
 		} else if a.GetSeries() != nil && b.GetSeries() == nil {