page_service: add benchmark for batching

[problame]

This PR adds two benchmark to demonstrate the effect of server-side getpage request batching added in https://github.com/neondatabase/neon/pull/9321.

For the CPU usage, I found the the prometheus crate's built-in CPU usage accounts the seconds at integer granularity. That's not enough you reduce the target benchmark runtime for local iteration. So, add a new libmetrics metric and report that.

The benchmarks are disabled because on our benchmark nodes, timer resolution isn't high enough. They work (no statement about quality) on my bare-metal devbox.

They will be refined and enabled once we find a fix. Candidates at time of writing are:

• https://github.com/neondatabase/neon/pull/9822
• https://github.com/neondatabase/neon/pull/9851

Refs:

• Epic: https://github.com/neondatabase/neon/issues/9376
• Extracted from https://github.com/neondatabase/neon/pull/9792

[github-actions[bot]]

5535 tests run: 5318 passed, 0 failed, 217 skipped (full report)

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Flaky tests (1)

#### Postgres 17 - `test_create_churn_during_restart`: [debug-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9820/11971860834/index.html#suites/ec4311502db344eee91f1354e9dc839b/456f7450440fda1c/retries)

Code coverage* (full report)

• functions: 31.4% (7950 of 25345 functions)
• lines: 49.3% (63100 of 128036 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
3eba1babe267649f8cebefc91c236589db030548 at 2024-11-22T12:39:16.085Z :recycle:}