test

metrics/registry.go

@@ -341,9 +341,10 @@ func DefaultSample() metrics.Sample {
 }
 
 func (r *rootRegistry) registerMetric(name string, tags Tags) string {
-	// Copy the list of tags so subsequent mutations do not break the world
+	// Copy the list of tags so subsequent mutations (e.g. sort, append) do not affect other metrics.
 	tagsCopy := make(Tags, len(tags))
 	copy(tagsCopy, tags)
+
 	metricID := toMetricTagsID(name, tagsCopy)
 	r.idToMetricMutex.Lock()
 	r.idToMetricWithTags[metricID] = metricWithTags{

metrics/registry_test.go

@@ -73,6 +73,17 @@ func TestMetricsWithTags(t *testing.T) {
 	assert.Equal(t, wantTags, gotTags)
 }
 
+func TestMetricDoesNotMutateInputTagSlice(t *testing.T) {
+	root := metrics.NewRootMetricsRegistry()
+
+	unsortedTags := metrics.Tags{metrics.MustNewTag("b", "b"), metrics.MustNewTag("a", "a")}
+
+	root.Counter("my-counter", unsortedTags...).Inc(1)
+
+	assert.Equal(t, metrics.Tags{metrics.MustNewTag("b", "b"), metrics.MustNewTag("a", "a")}, unsortedTags)
+}
+
+
 // Prefix should be used as provided (no case conversion/normalization), while tags should always be converted to
 // lowercase.
 func TestMetricsCasing(t *testing.T) {