feat(pageserver): do not read past image layers for vectored get

[skyzh]

Problem

Part of #7462

On metadata keyspace, vectored get will not stop if a key is not found, and will read past the image layer. However, the semantics is different from single get, because if a key does not exist in the image layer, it means that the key does not exist in the past, or have been deleted. This pull request fixed it by recording image layer coverage during the vectored get process and stop when the full keyspace is covered by an image layer. A corresponding test case is added to ensure generating image layer reduces the number of delta layers.

This optimization (or bug fix) also applies to rel block keyspaces. If a key is missing, we can know it's missing once the first image layer is reached. Page server will not attempt to read lower layers, which potentially incurs layer downloads + evictions.

Summary of changes

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]

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

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Flaky tests (1)

Postgres 16

• test_secondary_background_downloads: debug

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
d72c0da at 2024-05-20T17:45:43.192Z :recycle:}

[VladLazar]

Also, this change to vectored get definitely needs a test which validates we return a missing key error.

[skyzh]

added two test cases for missing key behavior on metadata + data keys. I have to admit it is too hard to generate custom image layers now. Tried several combinations of operation sequences and finally worked around duplicated layer panics.

[skyzh]

resolved the comments from the discussion. KeySpaceRandomAccum for image layer coverage, and removed the 'outer thing.

[skyzh]

ready for review :)

[VladLazar]

Approving with a few comments. I would let this soak in staging next week personally.