Add support for pg_stat_replication.(write_lag|flush_lag|replay_lag)

Add support for pg_stat_replication.(write_lag|flush_lag|replay_lag)

Use case. Why is this important?

For simple primary:replica setups it would be convenient to be able to monitor standby lag in seconds from the primary. pg_stat_replication already includes this information on postgres>=10, but the exporter does not parse it. AFAIK, workarounds would include

• monitoring byte offset from pg_stat_replication.write_lsn|flush_lsn|replay_lsn), but this does not quite capture issues where the replica has not replayed a business critical change however small in bytes.
• monitoring delay on the replica, but this would require spinning up a separate monitor only to monitor a single value on the replica.

Notes

I'm assuming the exporter does not support parsing the interval data type, which is why these metrics are marked as DISCARD here. I wonder if the following, explicit approach would be appropriate in this case:

diff --git a/cmd/postgres_exporter/queries.go b/cmd/postgres_exporter/queries.go
index fa0b5c2..e28d7b4 100644
--- a/cmd/postgres_exporter/queries.go
+++ b/cmd/postgres_exporter/queries.go
@@ -53,7 +53,10 @@ var queryOverrides = map[string][]OverrideQuery{
 			SELECT *,
 				(case pg_is_in_recovery() when 't' then null else pg_current_wal_lsn() end) AS pg_current_wal_lsn,
 				(case pg_is_in_recovery() when 't' then null else pg_wal_lsn_diff(pg_current_wal_lsn(), pg_lsn('0/0'))::float end) AS pg_current_wal_lsn_bytes,
-				(case pg_is_in_recovery() when 't' then null else pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn)::float end) AS pg_wal_lsn_diff
+				(case pg_is_in_recovery() when 't' then null else pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn)::float end) AS pg_wal_lsn_diff,
+				(case pg_is_in_recovery() when 't' then null else  extract(epoch from write_lag) end) as write_lag_seconds,
+				(case pg_is_in_recovery() when 't' then null else  extract(epoch from flush_lag) end) as flush_lag_seconds,
+				(case pg_is_in_recovery() when 't' then null else  extract(epoch from replay_lag) end) as replay_lag_seconds
 			FROM pg_stat_replication
 			`,
 		},

That way the deviation from the official fields is not in conflict with the source data.

Start in cmd/postgres_exporter/queries.go at the pg_stat_replication query override, then inspect the DISCARD handling referenced in cmd/postgres_exporter/postgres_exporter.go. Confirm how the write_lag, flush_lag, and replay_lag interval values are parsed and exposed; done means the three lag metrics are available in seconds for supported PostgreSQL versions.