Add query for 'pg_replication_slots' (prometheus-community#465)

The existing 'pg_stat_replication' data does not
include stats for inactive replication slots. This
commit adds a minimal amount of metrics from
'pg_replication_slots' to know if a slot is
active and its lag.

This is helpful to detect if an inactive slot
is causing the server to run out of storage due
to an inactive slot blocking WAL flushing.

Author: kppullin

cmd/postgres_exporter/postgres_exporter.go

@@ -315,6 +315,16 @@ var builtinMetricMaps = map[string]intermediateMetricMap{
 		true,
 		0,
 	},
+	"pg_replication_slots": {
+		map[string]ColumnMapping{
+			"slot_name":       {LABEL, "Name of the replication slot", nil, nil},
+			"database":        {LABEL, "Name of the database", nil, nil},
+			"active":          {GAUGE, "Flag indicating if the slot is active", nil, nil},
+			"pg_wal_lsn_diff": {GAUGE, "Replication lag in bytes", nil, nil},
+		},
+		true,
+		0,
+	},
 	"pg_stat_archiver": {
 		map[string]ColumnMapping{
 			"archived_count":     {COUNTER, "Number of WAL files that have been successfully archived", nil, nil},
@@ -407,6 +417,16 @@ var queryOverrides = map[string][]OverrideQuery{
 		},
 	},
 
+	"pg_replication_slots": {
+		{
+			semver.MustParseRange(">=9.4.0"),
+			`
+			SELECT slot_name, database, active, pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)
+			FROM pg_replication_slots
+			`,
+		},
+	},
+
 	"pg_stat_archiver": {
 		{
 			semver.MustParseRange(">=0.0.0"),