Fix #512

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/registry.go

@@ -872,7 +872,13 @@ func checkMetricConsistency(
 	h = hashAddByte(h, separatorByte)
 	// Make sure label pairs are sorted. We depend on it for the consistency
 	// check.
-	sort.Sort(labelPairSorter(dtoMetric.Label))
+	if !sort.IsSorted(labelPairSorter(dtoMetric.Label)) {
+		// We cannot sort dtoMetric.Label in place as it is immutable by contract.
+		copiedLabels := make([]*dto.LabelPair, len(dtoMetric.Label))
+		copy(copiedLabels, dtoMetric.Label)
+		sort.Sort(labelPairSorter(copiedLabels))
+		dtoMetric.Label = copiedLabels
+	}
 	for _, lp := range dtoMetric.Label {
 		h = hashAdd(h, lp.GetName())
 		h = hashAddByte(h, separatorByte)