Fix TestScrapeLoopDiscardDuplicateLabels test

This test calls Rollback() which is normally called from within append code.
Doing so means that staleness tracking data is outdated and need to by cycled manually.

Signed-off-by: Łukasz Mierzwa <l.mierzwa@gmail.com>

scrape/scrape_test.go

@@ -2636,6 +2636,9 @@ func TestScrapeLoopDiscardDuplicateLabels(t *testing.T) {
 	_, _, _, err := sl.append(slApp, []byte("test_metric{le=\"500\"} 1\ntest_metric{le=\"600\",le=\"700\"} 1\n"), "", time.Time{})
 	require.Error(t, err)
 	require.NoError(t, slApp.Rollback())
+	// We need to cycle staleness cache maps after a manual rollback. Otherwise they will have old entries in them,
+	// which would cause ErrDuplicateSampleForTimestamp errors on the next append.
+	sl.cache.iterDone(true)
 
 	q, err := s.Querier(time.Time{}.UnixNano(), 0)
 	require.NoError(t, err)