PG: Add recovery prefetch metrics

[bonnefoa]

What does this PR do?

Add Postgres recovery prefetch metrics.

Motivation

PG15 introduces asynchronous IO in the recovery process based on posix_fadvise (mostly using POSIX_FADV_WILLNEED). With this, pg_stat_recovery_prefetch was also introduced to provide visibility on how efficient prefetching is. Providing metrics will help to tune maintenance_io_concurrency parameter. For example, io_depth is saturated but the disk still had capacity so increasing maintenance_io_concurrency could help increase recovery throughput.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.32%. Comparing base (4d7826b) to head (3c6e88b).
Report is 1 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
confluent_platform	?
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
postgres	92.80% <100.00%> (+3.09%)	⬆️
presto	?
solr	?
tomcat	?
weblogic	?

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