lib/promrelabel: add tests for labelsToString() function

lib/promrelabel/relabel.go

@@ -446,7 +446,7 @@ func labelsToString(labels []prompbmarshal.Label) string {
 			break
 		}
 	}
-	if len(labelsCopy) <= 1 {
+	if mname != "" && len(labelsCopy) <= 1 {
 		return mname
 	}
 	b := []byte(mname)

lib/promrelabel/relabel_test.go

@@ -7,6 +7,53 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/prompbmarshal"
 )
 
+func TestLabelsToString(t *testing.T) {
+	f := func(labels []prompbmarshal.Label, sExpected string) {
+		t.Helper()
+		s := labelsToString(labels)
+		if s != sExpected {
+			t.Fatalf("unexpected result;\ngot\n%s\nwant\n%s", s, sExpected)
+		}
+	}
+	f(nil, "{}")
+	f([]prompbmarshal.Label{
+		{
+			Name:  "__name__",
+			Value: "foo",
+		},
+	}, "foo")
+	f([]prompbmarshal.Label{
+		{
+			Name:  "foo",
+			Value: "bar",
+		},
+	}, `{foo="bar"}`)
+	f([]prompbmarshal.Label{
+		{
+			Name:  "foo",
+			Value: "bar",
+		},
+		{
+			Name:  "a",
+			Value: "bc",
+		},
+	}, `{a="bc",foo="bar"}`)
+	f([]prompbmarshal.Label{
+		{
+			Name:  "foo",
+			Value: "bar",
+		},
+		{
+			Name:  "__name__",
+			Value: "xxx",
+		},
+		{
+			Name:  "a",
+			Value: "bc",
+		},
+	}, `xxx{a="bc",foo="bar"}`)
+}
+
 func TestApplyRelabelConfigs(t *testing.T) {
 	f := func(config string, labels []prompbmarshal.Label, isFinalize bool, resultExpected []prompbmarshal.Label) {
 		t.Helper()