Postgres metrics for stuck getpage requests

[myrrc]

#10327
Resolves: #11720

New metrics:

• compute_getpage_stuck_requests_total
• compute_getpage_max_inflight_stuck_time_ms

[github-actions]

If this PR added a GUC in the Postgres fork or neon extension,
please regenerate the Postgres settings in the cloud repo:

make NEON_WORKDIR=path/to/neon/checkout \
  -C goapp/internal/shareddomain/postgres generate

If you're an external contributor, a Neon employee will assist in
making sure this step is done.

[github-actions]

8327 tests run: 7827 passed, 0 failed, 500 skipped (full report)

---

Flaky tests (2)

Postgres 17

• test_timeline_size: debug-x86-64-without-lfc

Postgres 15

• test_pg_regress[v2-4]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.7% (8958 of 27399 functions)
• lines: 48.9% (78182 of 160031 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
56ab95c at 2025-04-28T18:43:20.006Z :recycle:}

[knizhnik]

I wonder why instead just calculating maximal time we can not just use histogram as for example file_cache_write_hist.
In this In this case we do not need separate counter metrics for stuck requests.

[myrrc]

I've considered this as an option, but our current latency histogram treats 10s as +Inf, so either adding a new bucket or another metrics.

Or you suggest turning max_ metric into a histogram? That's also an option but seems a bit extra for me, as we won't care for any but max values, and most of the histogram would just duplicated the exiting histogram for latencies