Support PostgreSQL 18

[pgloader]

Proposal

Use case. Why is this important?
There are changes in PostgreSQL 18. For example, checkpoint is moved out of pg_stat_bgwriter and into pg_stat_checkpoint. When postgres_exporter is going to support PostgreSQL 18 ?
“Nice to have” is not a good use case. :)

[sysadmind]

If there are things that don't work in 18, it would be more helpful to list what those things are. In general the project is compatible with all current versions.

[pgloader]

pg_stat_bgwriter_checkpoint_write_time_total returns empty
pg_stat_bgwriter_checkpoint_sync_time_total returns empty
pg_stat_bgwriter_checkpoints_timed_total returns empty
pg_stat_bgwriter_checkpoints_req_total return empty