Add comments to the sort by label functions

Signed-off-by: suntala <arati.rana@grafana.com>

promql/functions.go

@@ -406,17 +406,22 @@ func funcSortDesc(vals []parser.Value, args parser.Expressions, enh *EvalNodeHel
 
 // === sort_by_label(vector parser.ValueTypeVector, label parser.ValueTypeString...) (Vector, Annotations) ===
 func funcSortByLabel(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) (Vector, annotations.Annotations) {
+	// First, sort by the full label set. This ensures a consistent ordering in case sorting by the
+	// labels provided as arguments is not conclusive.
 	slices.SortFunc(vals[0].(Vector), func(a, b Sample) int {
 		return labels.Compare(a.Metric, b.Metric)
 	})
 
 	labels := stringSliceFromArgs(args[1:])
+	// Next, sort by the labels provided as arguments.
 	slices.SortFunc(vals[0].(Vector), func(a, b Sample) int {
-		// Iterate over each given label
+		// Iterate over each given label.
 		for _, label := range labels {
 			lv1 := a.Metric.Get(label)
 			lv2 := b.Metric.Get(label)
 
+			// If we encounter multiple samples with the same label values, the sorting which was
+			// performed in the first step will act as a "tie breaker".
 			if lv1 == lv2 {
 				continue
 			}
@@ -436,17 +441,22 @@ func funcSortByLabel(vals []parser.Value, args parser.Expressions, enh *EvalNode
 
 // === sort_by_label_desc(vector parser.ValueTypeVector, label parser.ValueTypeString...) (Vector, Annotations) ===
 func funcSortByLabelDesc(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) (Vector, annotations.Annotations) {
+	// First, sort by the full label set. This ensures a consistent ordering in case sorting by the
+	// labels provided as arguments is not conclusive.
 	slices.SortFunc(vals[0].(Vector), func(a, b Sample) int {
 		return labels.Compare(b.Metric, a.Metric)
 	})
 
 	labels := stringSliceFromArgs(args[1:])
+	// Next, sort by the labels provided as arguments.
 	slices.SortFunc(vals[0].(Vector), func(a, b Sample) int {
-		// Iterate over each given label
+		// Iterate over each given label.
 		for _, label := range labels {
 			lv1 := a.Metric.Get(label)
 			lv2 := b.Metric.Get(label)
 
+			// If we encounter multiple samples with the same label values, the sorting which was
+			// performed in the first step will act as a "tie breaker".
 			if lv1 == lv2 {
 				continue
 			}