feat: add metrics for pg_stat_checkpointer in v17+

[NiccoloFei]

Closes #3411

[github-actions[bot]]

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[gbartolini]

IMHO, this patch should have first fixed the issue for 17 then added pg_stat_checkpointer.

[NiccoloFei]

Because the query mapping needs to be unique (even though the 2 queries are mutually exclusive), I've distinguished the bgwriter metric's name in the following way:

• for PG17 and onward: pg_stat_background_writer
• for PG16 and lower: pg_stat_bgwriter (retaining the same name)

I've also thought about including and serving the 4 remaining bgwriter columns directly from the pg_stat_checkpointer query (on PG17+) but I figured it would be even more confusing this way, so I opted for the previous solution.

Tests running on: https://github.com/cloudnative-pg/postgres-trunk-containers/actions/runs/8881859306

[sxd]

/test depth=push limit=local test_level=4 feature_type=observability

[github-actions[bot]]

@sxd, here's the link to the E2E on CNPG workflow run: https://github.com/cloudnative-pg/cloudnative-pg/actions/runs/8939199640

[fcanovai]

I don't like the idea to generate metrics with different names from the same table, although on different postgres version.

Additionally, such changes should affect the grafana dashboard as well.

[mnencia]

Superseded by https://github.com/cloudnative-pg/cloudnative-pg/pull/4779

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