page_service: add benchmark for batching

[problame]

This PR adds two benchmark to demonstrate the effect of server-side
getpage request batching added in #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:

• WIP: page_service: higher-resolution timer for batching #9822
• page_service: rewrite batching to work without a timeout, pipeline in protocol handler instead #9851

Refs:

• Epic: Epic: get page throughput improvements #9376
• Extracted from page_service: getpage batching: refactor & minor fixes #9792

[github-actions]

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

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
3eba1ba at 2024-11-22T12:39:16.085Z :recycle:}