meter: add BuildLabels func that sorts and deletes duplicates

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

meter/meter.go

@@ -88,6 +88,16 @@ func (k byKey) Swap(i, j int) {
 	k[i*2+1], k[j*2+1] = k[j*2+1], k[i*2+1]
 }
 
+// BuildLables used to sort labels and delete duplicates.
+// Last value wins in case of duplicate label keys.
+func BuildLabels(labels ...string) []string {
+	if len(labels)%2 == 1 {
+		labels = labels[:len(labels)-1]
+	}
+	sort.Sort(byKey(labels))
+	return labels
+}
+
 // BuildName used to combine metric with labels.
 // If labels count is odd, drop last element
 func BuildName(name string, labels ...string) string {

meter/meter_test.go

@@ -14,12 +14,44 @@ func TestNoopMeter(t *testing.T) {
 	cnt.Inc()
 }
 
+func testEq(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestBuildLabels(t *testing.T) {
+	type testData struct {
+		src []string
+		dst []string
+	}
+
+	data := []testData{
+		testData{
+			src: []string{"zerolabel", "value3", "firstlabel", "value2"},
+			dst: []string{"firstlabel", "value2", "zerolabel", "value3"},
+		},
+	}
+
+	for _, d := range data {
+		if !testEq(d.dst, BuildLabels(d.src...)) {
+			t.Fatalf("slices not properly sorted: %v %v", d.dst, d.src)
+		}
+	}
+}
+
 func TestBuildName(t *testing.T) {
 	data := map[string][]string{
-		//	`my_metric{firstlabel="value2",zerolabel="value3"}`: []string{
-		//	"my_metric",
-		//			"zerolabel", "value3", "firstlabel", "value2",
-		//	},
+		`my_metric{firstlabel="value2",zerolabel="value3"}`: []string{
+			"my_metric",
+			"zerolabel", "value3", "firstlabel", "value2",
+		},
 		`my_metric{broker="broker2",register="mdns",server="tcp"}`: []string{
 			"my_metric",
 			"broker", "broker1", "broker", "broker2", "server", "http", "server", "tcp", "register", "mdns",