Incremenet getpage histogram in prefetch_lookup

[knizhnik]

Problem

PR #10442 added prefetch_lookup function.
It changed handling of getpage requests in compute.

Before:

1. Lookup in LFC (return if found)

2. Register prefetch buffer

3. Wait prefetch result (increment getpage_hist)
   Now:

4. Lookup prefetch ring (return if prefetch request is already completed)

5. Lookup in LFC (return if found)

6. Register prefetch buffer

7. Wait prefetch result (increment getpage_hist)

So if prefetch result is already available, then get page histogram is not incremented.
It case failure of some test_throughtput benchmarks:
https://neondb.slack.com/archives/C033RQ5SPDH/p1740425527249499

Summary of changes

Increment getpage histogram in prefetch_lookup

[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]

7711 tests run: 7335 passed, 0 failed, 376 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_subscriber_branching: debug-x86-64-without-lfc
• test_branched_from_many_empty_parents_size: debug-x86-64-without-lfc

Postgres 14

• test_branching_with_pgbench[flat-1-10]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.8% (8631 of 26298 functions)
• lines: 48.6% (72630 of 149426 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
1ccb40b at 2025-02-25T08:39:54.376Z :recycle:}

[skyzh]

Thanks!