produce warn message if non 2h block time

cmd/thanos/sidecar.go

@@ -336,10 +336,16 @@ func validatePrometheus(ctx context.Context, logger log.Logger, m *promMetadata)
 	if err != nil {
 		return errors.Wrap(err, "failed to check flags")
 	}
-	// Check if min-block-time and max-block-time are 2h.
-	if flags.TSDBMinTime != model.Duration(2*time.Hour) || flags.TSDBMaxTime != model.Duration(2*time.Hour) {
+
+	// Check if compaction is disabled.
+	if flags.TSDBMinTime != flags.TSDBMaxTime {
 		return errors.Errorf("Found that TSDB Max time is %s and Min time is %s. "+
-			"Compaction needs to be disabled (storage.tsdb.min-block-duration = storage.tsdb.max-block-duration = 2h)", flags.TSDBMaxTime, flags.TSDBMinTime)
+			"Compaction needs to be disabled (storage.tsdb.min-block-duration = storage.tsdb.max-block-duration)", flags.TSDBMaxTime, flags.TSDBMinTime)
+	}
+
+	// Check if block time is 2h.
+	if flags.TSDBMinTime != model.Duration(2*time.Hour) {
+		level.Warn(logger).Log("msg", "Found that TSDB block time is not 2h. Only 2h block time is recommended.", "block-time", flags.TSDBMinTime)
 	}
 
 	return nil