pageserver: improve read amp metric

[erikgrinaker]

Problem

The current global pageserver_layers_visited_per_vectored_read_global metric does not appear to accurately measure read amplification. It divides the layer count by the number of reads in a batch, but this means that e.g. 10 reads with 100 L0 layers will only measure a read amp of 10 per read, while the actual read amp was 100.

While the cost of layer visits are amortized across the batch, and some layers may not intersect with a given key, each visited layer contributes directly to the observed latency for every read in the batch, which is what we care about.

Touches https://github.com/neondatabase/cloud/issues/23283.
Extracted from #10566.

Summary of changes

• Count the number of layers visited towards each read in the batch, instead of the average across the batch.
• Rename pageserver_layers_visited_per_vectored_read_global to pageserver_layers_per_read_global.
• Reduce the read amp log warning threshold down from 512 to 100.

[github-actions]

7414 tests run: 7063 passed, 0 failed, 351 skipped (full report)

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Flaky tests (7)

Postgres 17

• test_pgdata_import_smoke[None-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: debug-x86-64-without-lfc, release-arm64-with-lfc, release-arm64-without-lfc
• test_pgdata_import_smoke[8-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: debug-x86-64-without-lfc, release-arm64-with-lfc, release-arm64-without-lfc

Postgres 14

• test_timeline_archive[4]: release-x86-64-with-lfc

Code coverage* (full report)

• functions: 33.4% (8508 of 25499 functions)
• lines: 49.1% (71435 of 145499 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7cfd38c at 2025-01-30T00:15:57.405Z :recycle:}