Add some more replication slot metrics

[save-buffer]

Problem

We want to add alerts for when people's replication slots break, and also metrics for retained WAL so that we can make warn customers when their storage gets bloated.

Summary of changes

Adds the metrics. Addresses #7593

[github-actions]

3078 tests run: 2951 passed, 0 failed, 127 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_timeline_deletion_with_files_stuck_in_upload_queue: debug

Postgres 14

• test_location_conf_churn[1]: release

Code coverage* (full report)

• functions: 31.4% (6412 of 20427 functions)
• lines: 48.1% (49276 of 102532 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c6bf781 at 2024-05-21T00:08:16.179Z :recycle:}