Fix: handle nil variableLabels in Desc.String() method and add tests for nil label values

Fixes #1684

Signed-off-by: Kemal Akkoyun <kakkoyun@gmail.com>

prometheus/desc.go

@@ -189,12 +189,15 @@ func (d *Desc) String() string {
 			fmt.Sprintf("%s=%q", lp.GetName(), lp.GetValue()),
 		)
 	}
-	vlStrings := make([]string, 0, len(d.variableLabels.names))
+	vlStrings := []string{}
-	for _, vl := range d.variableLabels.names {
+	if d.variableLabels != nil {
-		if fn, ok := d.variableLabels.labelConstraints[vl]; ok && fn != nil {
+		vlStrings = make([]string, 0, len(d.variableLabels.names))
-			vlStrings = append(vlStrings, fmt.Sprintf("c(%s)", vl))
+		for _, vl := range d.variableLabels.names {
-		} else {
+			if fn, ok := d.variableLabels.labelConstraints[vl]; ok && fn != nil {
-			vlStrings = append(vlStrings, vl)
+				vlStrings = append(vlStrings, fmt.Sprintf("c(%s)", vl))
+			} else {
+				vlStrings = append(vlStrings, vl)
+			}
 		}
 	}
 	return fmt.Sprintf(

prometheus/desc_test.go

@@ -28,3 +28,36 @@ func TestNewDescInvalidLabelValues(t *testing.T) {
 		t.Errorf("NewDesc: expected error because: %s", desc.err)
 	}
 }
+
+func TestNewDescNilLabelValues(t *testing.T) {
+	desc := NewDesc(
+		"sample_label",
+		"sample label",
+		nil,
+		nil,
+	)
+	if desc.err != nil {
+		t.Errorf("NewDesc: unexpected error: %s", desc.err)
+	}
+}
+
+func TestNewDescWithNilLabelValues_String(t *testing.T) {
+	desc := NewDesc(
+		"sample_label",
+		"sample label",
+		nil,
+		nil,
+	)
+	if desc.String() != `Desc{fqName: "sample_label", help: "sample label", constLabels: {}, variableLabels: {}}` {
+		t.Errorf("String: unexpected output: %s", desc.String())
+	}
+}
+
+func TestNewInvalidDesc_String(t *testing.T) {
+	desc := NewInvalidDesc(
+		nil,
+	)
+	if desc.String() != `Desc{fqName: "", help: "", constLabels: {}, variableLabels: {}}` {
+		t.Errorf("String: unexpected output: %s", desc.String())
+	}
+}