Fix duplicate tags when overriding tag (#4056)

metric/metric.go

@@ -123,6 +123,7 @@ func (m *metric) AddTag(key, value string) {
 
 		if key == tag.Key {
 			tag.Value = value
+			return
 		}
 
 		m.tags = append(m.tags, nil)

metric/metric_test.go

@@ -63,6 +63,7 @@ func TestAddTagOverwrites(t *testing.T) {
 	value, ok := m.GetTag("host")
 	require.True(t, ok)
 	require.Equal(t, "example.org", value)
+	require.Equal(t, 1, len(m.TagList()))
 }
 
 func TestRemoveTagNoEffectOnMissingTags(t *testing.T) {