scrape: add Target.LabelsRange

This allows users of a Target to iterate labels without allocating heap memory.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

scrape/target.go

@@ -181,6 +181,15 @@ func (t *Target) Labels() labels.Labels {
 	return b.Labels()
 }
 
+// LabelsRange calls f on each public label of the target.
+func (t *Target) LabelsRange(f func(l labels.Label)) {
+	t.labels.Range(func(l labels.Label) {
+		if !strings.HasPrefix(l.Name, model.ReservedLabelPrefix) {
+			f(l)
+		}
+	})
+}
+
 // DiscoveredLabels returns a copy of the target's labels before any processing.
 func (t *Target) DiscoveredLabels() labels.Labels {
 	t.mtx.Lock()

scrape/target_test.go

@@ -43,6 +43,17 @@ func TestTargetLabels(t *testing.T) {
 	want := labels.FromStrings(model.JobLabel, "some_job", "foo", "bar")
 	got := target.Labels()
 	require.Equal(t, want, got)
+	i := 0
+	target.LabelsRange(func(l labels.Label) {
+		switch i {
+		case 0:
+			require.Equal(t, labels.Label{Name: "foo", Value: "bar"}, l)
+		case 1:
+			require.Equal(t, labels.Label{Name: model.JobLabel, Value: "some_job"}, l)
+		}
+		i++
+	})
+	require.Equal(t, 2, i)
 }
 
 func TestTargetOffset(t *testing.T) {