pageserver: add disk consistent and remote lsn metrics

[VladLazar]

Problem

There's no metrics for disk consistent LSN and remote LSN. This stuff is useful when looking at ingest performance.

Summary of changes

Two per timeline metrics are added: pageserver_disk_consistent_lsn and pageserver_projected_remote_consistent_lsn. I went for the projected remote lsn instead of the visible one
because that more closely matches remote storage write tput. Ideally we would have both, but these metrics are expensive.

[github-actions]

6400 tests run: 6110 passed, 0 failed, 290 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_prefetch[None]: release-arm64

Code coverage* (full report)

• functions: 31.5% (8328 of 26475 functions)
• lines: 47.8% (65454 of 136924 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
4f74ea9 at 2024-12-05T19:40:11.653Z :recycle:}

[problame]

per-timeline metrics need to be cleaned up. Actually, some tests should be failing with the current state of the PR because there's not enough cleanup

[problame]

Approving in expectation that #10005 (comment) will be addressed