Reduce constrainLabels allocations

prometheus/vec.go

@@ -92,7 +92,7 @@ func (m *MetricVec) DeleteLabelValues(lvs ...string) bool {
 // This method is used for the same purpose as DeleteLabelValues(...string). See
 // there for pros and cons of the two methods.
 func (m *MetricVec) Delete(labels Labels) bool {
-	labels = constrainLabels(m.desc, labels)
+	constrainLabels(m.desc, labels)
 	h, err := m.hashLabels(labels)
 	if err != nil {
 		return false
@@ -108,7 +108,7 @@ func (m *MetricVec) Delete(labels Labels) bool {
 // Note that curried labels will never be matched if deleting from the curried vector.
 // To match curried labels with DeletePartialMatch, it must be called on the base vector.
 func (m *MetricVec) DeletePartialMatch(labels Labels) int {
-	labels = constrainLabels(m.desc, labels)
+	constrainLabels(m.desc, labels)
 	return m.metricMap.deleteByLabels(labels, m.curry)
 }
 
@@ -228,7 +228,7 @@ func (m *MetricVec) GetMetricWithLabelValues(lvs ...string) (Metric, error) {
 // around MetricVec, implementing a vector for a specific Metric implementation,
 // for example GaugeVec.
 func (m *MetricVec) GetMetricWith(labels Labels) (Metric, error) {
-	labels = constrainLabels(m.desc, labels)
+	constrainLabels(m.desc, labels)
 	h, err := m.hashLabels(labels)
 	if err != nil {
 		return nil, err
@@ -646,16 +646,12 @@ func inlineLabelValues(lvs []string, curry []curriedLabelValue) []string {
 	return labelValues
 }
 
-func constrainLabels(desc *Desc, labels Labels) Labels {
-	constrainedValues := make(Labels, len(labels))
+func constrainLabels(desc *Desc, labels Labels) {
 	for l, v := range labels {
 		if i, ok := indexOf(l, desc.variableLabels.labelNames()); ok {
-			constrainedValues[l] = desc.variableLabels[i].Constrain(v)
-			continue
+			labels[l] = desc.variableLabels[i].Constrain(v)
 		}
-		constrainedValues[l] = v
 	}
-	return constrainedValues
 }
 
 func constrainLabelValues(desc *Desc, lvs []string, curry []curriedLabelValue) []string {