Wrong metric name/location for pg_stat_replication_pg_current_wal_lsn_bytes

[kautkata]

What did you do?
I tried to figure out that actually get's returned by pg_stat_replication_pg_wal_lsn_diff metric, so I reviewed your code, to be precise

postgres_exporter/cmd/postgres_exporter/queries.go

Line 53 in ddd5136

SELECT *,

What did you expect to see?
Replication related data gathering.

What did you see instead? Under which circumstances?
Total byte count written in wal over database lifetime.
PostgreSQL documentation states:

pg_wal_lsn_diff ( lsn1 pg_lsn, lsn2 pg_lsn ) → numeric
Calculates the difference in bytes (lsn1 - lsn2) between two write-ahead log locations. This can be used with pg_stat_replication or some of the functions shown in Table 9.87 to get the replication lag.

and pg_lsn('0/0') in essence will return beginning of the wal so it's total bytes written in the wall. Metric it self is sane but it's not related to replication and it could/should be gathered for all database instances despite what pg_is_in_recovery() states

Environment
N/A

• System information: Linux 5.10.167-147.601.amzn2.x86_64 x86_64

• postgres_exporter version issues applies to latest version as well:

postgres_exporter, version 0.11.1 (branch: HEAD, revision: b5fd246)
build user: root@a2c981f603ff
build date: 20220818-20:58:17
go version: go1.18.5
platform: linux/amd64

• postgres_exporter flags:
  N/A

• PostgreSQL version: 14.6

• Logs:
  N/A