Update prometheus/histogram.go

Co-authored-by: Björn Rabenstein <github@rabenste.in>
Signed-off-by: Ziqi Zhao <zhaoziqi9146@gmail.com>

prometheus/histogram.go

@@ -1659,8 +1659,7 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
 
 	// When the number of exemplars exceeds the limit, remove one exemplar.
 	var (
-		// The index where to remove the old exemplar.
-		rIdx int
+		rIdx int // The index where to remove the old exemplar.
 
 		ot    = time.Now() // Oldest timestamp seen.
 		otIdx = -1         // Index of the exemplar with the oldest timestamp.