pageserver: add relsize cache metrics

[erikgrinaker]

Problem

We don't have any observability for the relation size cache. We have seen cache misses cause significant performance impact with high relation counts.

Touches #9855.

Summary of changes

Adds the following metrics:

• pageserver_relsize_cache_entries
• pageserver_relsize_cache_hits
• pageserver_relsize_cache_misses
• pageserver_relsize_cache_misses_old

[github-actions]

6941 tests run: 6633 passed, 0 failed, 308 skipped (full report)

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Code coverage* (full report)

• functions: 30.7% (7982 of 26019 functions)
• lines: 48.6% (63353 of 130437 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6669986 at 2024-11-27T13:51:18.524Z :recycle:}