pageserver: add read depth metrics and test

[VladLazar]

Problem

We recently went through an incident where compaction was inhibited by a bug.
We didn't observe this until quite late because we did not have alerting on deep reads.

Summary of changes

• Tweak an existing metric that tracks the depth of a read on the non-vectored read path:
  • Give it a better name
  • Track all layers
  • Larger buckets
• Add a similar metric for the vectored read path
• Add a compaction smoke test which uses these metrics. This test would have caught
  the compaction issue mentioned earlier.

Once this merges and we have a better understanding for where the upper bound is
merge these warnings to aid debugging: #7465

Related #7428

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

[github-actions]

2778 tests run: 2657 passed, 0 failed, 121 skipped (full report)

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Code coverage* (full report)

• functions: 28.1% (6470 of 23045 functions)
• lines: 46.8% (45704 of 97615 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
ee2de0a at 2024-04-22T17:23:05.758Z :recycle:}