addressing comments

Signed-off-by: Zachary Caldarola <zachary.caldarola@reddit.com>

Author: Zachary Caldarola

collector/pg_replication_slots.go

@@ -1,4 +1,4 @@
-// Copyright 2022 The Prometheus Authors
+// Copyright 2023 The Prometheus Authors
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
@@ -34,9 +34,14 @@ func NewPGReplicationSlotCollector(logger log.Logger) (Collector, error) {
 }
 
 var pgReplicationSlot = map[string]*prometheus.Desc{
-"lsn_distance": prometheus.NewDesc(
-"pg_replication_slot_lsn_distance",
-"Disk space used by the database",
+"current_wal_lsn": prometheus.NewDesc(
+"pg_replication_slot_current_wal_lsn",
+"current wal lsn value",
+[]string{"slot_name"}, nil,
+),
+"confirmed_flush_lsn": prometheus.NewDesc(
+"pg_replication_slot_confirmed_flush_lsn",
+"last lsn confirmed flushed to the replication slot",
 []string{"slot_name"}, nil,
 ),
 }
@@ -45,7 +50,8 @@ func (PGReplicationSlotCollector) Update(ctx context.Context, db *sql.DB, ch cha
 rows, err := db.QueryContext(ctx,
 `SELECT
 slot_name,
-(pg_current_wal_lsn() - confirmed_flush_lsn) AS lsn_distance
+pg_current_wal_lsn() AS current_wal_lsn,
+confirmed_flush_lsn
 FROM
 pg_replication_slots;`)
 if err != nil {
@@ -55,14 +61,19 @@ func (PGReplicationSlotCollector) Update(ctx context.Context, db *sql.DB, ch cha
 
 for rows.Next() {
 var slot_name string
-var size int64
-if err := rows.Scan(&slot_name, &size); err != nil {
+var wal_lsn int64
+var flush_lsn int64
+if err := rows.Scan(&slot_name, &wal_lsn, &flush_lsn); err != nil {
 return err
 }
 
 ch <- prometheus.MustNewConstMetric(
-pgReplicationSlot["size_bytes"],
-prometheus.GaugeValue, float64(size), slot_name,
+pgReplicationSlot["current_wal_lsn"],
+prometheus.GaugeValue, float64(wal_lsn), slot_name,
+)
+ch <- prometheus.MustNewConstMetric(
+pgReplicationSlot["confirmed_flush_lsn"],
+prometheus.GaugeValue, float64(flush_lsn), slot_name,
 )
 }
 if err := rows.Err(); err != nil {