Export last replay age in replication collector

The exported replication lag does not handle all failure modes, and can
report 0 for replicas that are out of sync and incapable of recovery.

A proper replacement for that metric would require a different approach
(see e.g. prometheus-community#1007), but for a lot of folks, simply exporting the age of
the last replay can provide a pretty strong signal for something being
amiss.

I think this solution might be preferable to prometheus-community#977, though the lag
metric needs to be fixed or abandoned eventually.

Signed-off-by: Conrad Hoffmann <ch@bitfehler.net>

collector/pg_replication.go

@@ -51,6 +51,15 @@ var (
 		"Indicates if the server is a replica",
 		[]string{}, nil,
 	)
+	pgReplicationLastReplay = prometheus.NewDesc(
+		prometheus.BuildFQName(
+			namespace,
+			replicationSubsystem,
+			"last_replay_seconds",
+		),
+		"Age of last replay in seconds",
+		[]string{}, nil,
+	)
 
 	pgReplicationQuery = `SELECT
 	CASE
@@ -61,7 +70,8 @@ var (
 	CASE
 		WHEN pg_is_in_recovery() THEN 1
 		ELSE 0
-	END as is_replica`
+	END as is_replica,
+	GREATEST (0, EXTRACT(EPOCH FROM (now() - pg_last_xact_replay_timestamp()))) as last_replay`
 )
 
 func (c *PGReplicationCollector) Update(ctx context.Context, instance *instance, ch chan<- prometheus.Metric) error {
@@ -72,7 +82,8 @@ func (c *PGReplicationCollector) Update(ctx context.Context, instance *instance,
 
 	var lag float64
 	var isReplica int64
-	err := row.Scan(&lag, &isReplica)
+	var replayAge float64
+	err := row.Scan(&lag, &isReplica, &replayAge)
 	if err != nil {
 		return err
 	}
@@ -84,5 +95,9 @@ func (c *PGReplicationCollector) Update(ctx context.Context, instance *instance,
 		pgReplicationIsReplica,
 		prometheus.GaugeValue, float64(isReplica),
 	)
+	ch <- prometheus.MustNewConstMetric(
+		pgReplicationLastReplay,
+		prometheus.GaugeValue, replayAge,
+	)
 	return nil
 }

collector/pg_replication_test.go

@@ -31,9 +31,9 @@ func TestPgReplicationCollector(t *testing.T) {
 
 	inst := &instance{db: db}
 
-	columns := []string{"lag", "is_replica"}
+	columns := []string{"lag", "is_replica", "last_replay"}
 	rows := sqlmock.NewRows(columns).
-		AddRow(1000, 1)
+		AddRow(1000, 1, 3)
 	mock.ExpectQuery(sanitizeQuery(pgReplicationQuery)).WillReturnRows(rows)
 
 	ch := make(chan prometheus.Metric)