Prometheus: Reduce memory allocations in QueryData.processExemplars (#63373)

* tsdb/prometheus/querydata: Reduce memory allocations in QueryData.processExemplars

---------

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/tsdb/prometheus/querydata/response.go

@@ -78,14 +78,15 @@ func (s *QueryData) processExemplars(q *models.Query, dr backend.DataResponse) b
 
 		seriesLabels := getSeriesLabels(frame)
 		labelTracker.Add(seriesLabels)
+		labelTracker.AddFields(frame.Fields[2:])
 		for rowIdx := 0; rowIdx < frame.Fields[0].Len(); rowIdx++ {
-			row := frame.RowCopy(rowIdx)
-			labels := getLabels(frame, row)
-			labelTracker.Add(labels)
+			ts := frame.CopyAt(0, rowIdx).(time.Time)
+			val := frame.CopyAt(1, rowIdx).(float64)
 			ex := models.Exemplar{
-				Labels:       labels,
-				Value:        row[1].(float64),
-				Timestamp:    row[0].(time.Time),
+				RowIdx:       rowIdx,
+				Fields:       frame.Fields[2:],
+				Value:        val,
+				Timestamp:    ts,
 				SeriesLabels: seriesLabels,
 			}
 			sampler.Add(ex)
@@ -200,11 +201,3 @@ func getSeriesLabels(frame *data.Frame) data.Labels {
 	// series labels are stored on the value field (index 1)
 	return frame.Fields[1].Labels.Copy()
 }
-
-func getLabels(frame *data.Frame, row []interface{}) map[string]string {
-	labels := make(map[string]string)
-	for i := 2; i < len(row); i++ {
-		labels[frame.Fields[i].Name] = row[i].(string)
-	}
-	return labels
-}