feat(pageserver): add metrics for number of valid leases after each refresh

[yliang412]

Part of #7497, closes #8120.

Summary of changes

This PR adds a metric to track the number of valid leases after GCInfo gets refreshed each time.

Besides this metric, we should also track disk space and synthetic size (after #8071 is closed) to make sure leases are used properly.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[skyzh]

Per-timeline metrics should be cleaned-up after the timeline gets detached. Please refer to other timeline metrics and copy-paste the cleanup code. e2e tests also need to be changed to include the name of this new metrics, but I forget which regress test needs to be changed. You can wait the CI run and see what to fix :)

[github-actions]

2922 tests run: 2805 passed, 0 failed, 117 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_isolation[None]: debug

Postgres 14

• test_subscriber_restart: release

Code coverage* (full report)

• functions: 32.5% (6876 of 21137 functions)
• lines: 50.2% (53658 of 106990 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7c6b284 at 2024-06-25T15:37:17.629Z :recycle:}