Fix swapped flushedLsn and receiveStartTli for wal_receiver collector (#1198)

In `pgStatWalReceiverQueryTemplate`, the order of the columns (when `hasFlushedLSN == true`) is:

- ...
- `receive_start_lsn`
- `flushed_lsn`
- `receive_start_tli`
- ...

However, columns were scanned in this order:

- ...
- `receive_start_lsn` -> `receiveStartLsn`
- `receive_start_tli` -> `flushedLsn` (!)
- `flushed_lsn` -> `receiveStartTli` (!)
- ...

This incorrect hydration of variables also manifests as swapped values for the
`pg_stat_wal_receiver_flushed_lsn` and `pg_stat_wal_receiver_receive_start_tli` metrics.

This seems to be a bug that has existed since the initial implementation:

- 2d7e152751
- https://github.com/prometheus-community/postgres_exporter/pull/844

In this patch, I'm:

- fixing the `.Scan()`, so that it hydrates variables in the correct order

- adjusting the order in which metrics are pushed out to the channel,
  to follow the order we consume them in
  (.., `receive_start_lsn`, `flushed_lsn`, `receive_start_tli`, ..)

- adjusting the walreceiver tests, to follow the new order (which matches .`Scan()`)

- fixing a small identation issue in `pgStatWalReceiverQueryTemplate`

Signed-off-by: Slavi Pantaleev <slavi@devture.com>

collector/pg_stat_walreceiver.go

@@ -108,7 +108,7 @@ var (
 		status,
 		(receive_start_lsn- '0/0') %% (2^52)::bigint as receive_start_lsn,
 		%s
-receive_start_tli,
+		receive_start_tli,
 		received_tli,
 		extract(epoch from last_msg_send_time) as last_msg_send_time,
 		extract(epoch from last_msg_receipt_time) as last_msg_receipt_time,
@@ -147,7 +147,7 @@ func (c *PGStatWalReceiverCollector) Update(ctx context.Context, instance *insta
 		var lastMsgSendTime, lastMsgReceiptTime, latestEndTime sql.NullFloat64
 
 		if hasFlushedLSN {
-			if err := rows.Scan(&upstreamHost, &slotName, &status, &receiveStartLsn, &receiveStartTli, &flushedLsn, &receivedTli, &lastMsgSendTime, &lastMsgReceiptTime, &latestEndLsn, &latestEndTime, &upstreamNode); err != nil {
+			if err := rows.Scan(&upstreamHost, &slotName, &status, &receiveStartLsn, &flushedLsn, &receiveStartTli, &receivedTli, &lastMsgSendTime, &lastMsgReceiptTime, &latestEndLsn, &latestEndTime, &upstreamNode); err != nil {
 				return err
 			}
 		} else {
@@ -209,12 +209,6 @@ func (c *PGStatWalReceiverCollector) Update(ctx context.Context, instance *insta
 			float64(receiveStartLsn.Int64),
 			labels...)
 
-		ch <- prometheus.MustNewConstMetric(
-			statWalReceiverReceiveStartTli,
-			prometheus.GaugeValue,
-			float64(receiveStartTli.Int64),
-			labels...)
-
 		if hasFlushedLSN {
 			ch <- prometheus.MustNewConstMetric(
 				statWalReceiverFlushedLSN,
@@ -223,6 +217,12 @@ func (c *PGStatWalReceiverCollector) Update(ctx context.Context, instance *insta
 				labels...)
 		}
 
+		ch <- prometheus.MustNewConstMetric(
+			statWalReceiverReceiveStartTli,
+			prometheus.GaugeValue,
+			float64(receiveStartTli.Int64),
+			labels...)
+
 		ch <- prometheus.MustNewConstMetric(
 			statWalReceiverReceivedTli,
 			prometheus.GaugeValue,

collector/pg_stat_walreceiver_test.go

@@ -50,8 +50,8 @@ func TestPGStatWalReceiverCollectorWithFlushedLSN(t *testing.T) {
 		"slot_name",
 		"status",
 		"receive_start_lsn",
-		"receive_start_tli",
 		"flushed_lsn",
+		"receive_start_tli",
 		"received_tli",
 		"last_msg_send_time",
 		"last_msg_receipt_time",
@@ -65,8 +65,8 @@ func TestPGStatWalReceiverCollectorWithFlushedLSN(t *testing.T) {
 			"bar",
 			"stopping",
 			int64(1200668684563608),
-			1687321285,
 			int64(1200668684563609),
+			1687321285,
 			1687321280,
 			1687321275,
 			1687321276,
@@ -88,8 +88,8 @@ func TestPGStatWalReceiverCollectorWithFlushedLSN(t *testing.T) {
 	}()
 	expected := []MetricResult{
 		{labels: labelMap{"upstream_host": "foo", "slot_name": "bar", "status": "stopping"}, value: 1200668684563608, metricType: dto.MetricType_COUNTER},
-		{labels: labelMap{"upstream_host": "foo", "slot_name": "bar", "status": "stopping"}, value: 1687321285, metricType: dto.MetricType_GAUGE},
 		{labels: labelMap{"upstream_host": "foo", "slot_name": "bar", "status": "stopping"}, value: 1200668684563609, metricType: dto.MetricType_COUNTER},
+		{labels: labelMap{"upstream_host": "foo", "slot_name": "bar", "status": "stopping"}, value: 1687321285, metricType: dto.MetricType_GAUGE},
 		{labels: labelMap{"upstream_host": "foo", "slot_name": "bar", "status": "stopping"}, value: 1687321280, metricType: dto.MetricType_GAUGE},
 		{labels: labelMap{"upstream_host": "foo", "slot_name": "bar", "status": "stopping"}, value: 1687321275, metricType: dto.MetricType_COUNTER},
 		{labels: labelMap{"upstream_host": "foo", "slot_name": "bar", "status": "stopping"}, value: 1687321276, metricType: dto.MetricType_COUNTER},