prometheus: fix copying of metrics labels (scionproto#4450)

When copying metric labels we assumed that `append` will always
create a copy of the label array. This is not necessarily the case.
In such case two metrics may end up with the same underlaying
array of labels and change to one of them also overwrites the
labels in the other one.

Author: sustrik

pkg/metrics/prometheus.go

@@ -91,7 +91,9 @@ func (lvs labelValuesSlice) With(labelValues ...string) labelValuesSlice {
 	if len(labelValues)%2 != 0 {
 		labelValues = append(labelValues, "unknown")
 	}
-	return append(lvs, labelValues...)
+	result := make(labelValuesSlice, len(lvs))
+	copy(result, lvs)
+	return append(result, labelValues...)
 }
 
 // gauge implements Gauge, via a Prometheus GaugeVec.