Follow-up to #13060: Add test to ensure staleness tracking

This commit introduces an additional test in `scrape_test.go` to verify
staleness tracking when `trackTimestampStaleness` is enabled. The new
`TestScrapeLoopAppendStalenessIfTrackTimestampStaleness` function
asserts that the scrape loop correctly appends staleness markers when
necessary, reflecting the expected behavior with the feature flag turned
on.

The previous tests were only testing end of scrape staleness.

Signed-off-by: Julien Pivotto <roidelapluie@o11y.eu>

scrape/scrape_test.go

@@ -2038,6 +2038,57 @@ func TestScrapeLoopAppendNoStalenessIfTimestamp(t *testing.T) {
 	require.Equal(t, want, app.resultFloats, "Appended samples not as expected:\n%s", appender)
 }
 
+func TestScrapeLoopAppendStalenessIfTrackTimestampStaleness(t *testing.T) {
+	app := &collectResultAppender{}
+	sl := newScrapeLoop(context.Background(),
+		nil, nil, nil,
+		nopMutator,
+		nopMutator,
+		func(ctx context.Context) storage.Appender { return app },
+		nil,
+		0,
+		true,
+		true,
+		0, 0,
+		nil,
+		0,
+		0,
+		false,
+		false,
+		false,
+		nil,
+		false,
+		newTestScrapeMetrics(t),
+	)
+
+	now := time.Now()
+	slApp := sl.appender(context.Background())
+	_, _, _, err := sl.append(slApp, []byte("metric_a 1 1000\n"), "", now)
+	require.NoError(t, err)
+	require.NoError(t, slApp.Commit())
+
+	slApp = sl.appender(context.Background())
+	_, _, _, err = sl.append(slApp, []byte(""), "", now.Add(time.Second))
+	require.NoError(t, err)
+	require.NoError(t, slApp.Commit())
+
+	// DeepEqual will report NaNs as being different, so replace with a different value.
+	app.resultFloats[1].f = 42
+	want := []floatSample{
+		{
+			metric: labels.FromStrings(model.MetricNameLabel, "metric_a"),
+			t:      1000,
+			f:      1,
+		},
+		{
+			metric: labels.FromStrings(model.MetricNameLabel, "metric_a"),
+			t:      timestamp.FromTime(now.Add(time.Second)),
+			f:      42,
+		},
+	}
+	require.Equal(t, want, app.resultFloats, "Appended samples not as expected:\n%s", appender)
+}
+
 func TestScrapeLoopAppendExemplar(t *testing.T) {
 	tests := []struct {
 		title                   string