add benchmark demonstrating metrics/prometheus crate multicore scalability pitfalls & workarounds

[problame]

We use the metrics / prometheus crate in the Pageserver code base.

This PR demonstrates

• typical performance pitfalls with that crate
• our current set of techniques to avoid most of these pitfalls.

refs

• pageserver: determine metrics CPU overhead of metrics #10948
• impr: propagate VirtualFile metrics via RequestContext #7202
  • I applied the label_values__cache_label_values_lookup technique there.
  • It didn't yield measurable results in high-level benchmarks though.

[github-actions]

7744 tests run: 7366 passed, 0 failed, 378 skipped (full report)

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Code coverage* (full report)

• functions: 32.8% (8644 of 26359 functions)
• lines: 48.7% (73207 of 150476 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
03c5938 at 2025-02-27T14:18:18.930Z :recycle:}