Maintain number of used pages for LFC

[knizhnik]

Problem

LFC cache entry is chunk (right now size of chunk is 1Mb). LFC statistics shows number of chunks, but not number of used pages. And autoscaling team wants to know how sparse LFC is:
https://neondb.slack.com/archives/C04DGM6SMTM/p1726782793595969
It is possible to obtain it from the view select count(*) from local_cache.
Nut it is expensive operation, enumerating all entries in LFC under lock.

Summary of changes

This PR added "file_cache_used_pages" to neon_lfc_stats view:

 select * from neon_lfc_stats;
        lfc_key        | lfc_value 
-----------------------+-----------
 file_cache_misses     |   3139029
 file_cache_hits       |   4098394
 file_cache_used       |      1024
 file_cache_writes     |   3173728
 file_cache_size       |      1024
 file_cache_used_pages |     25689
(6 rows)

Please notice that this PR doesn't change neon extension API, so no need to create new version of Neon extension.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

4977 tests run: 4813 passed, 0 failed, 164 skipped (full report)

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Flaky tests (5)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_pg_regress[None]: debug-x86-64

Postgres 16

• test_location_conf_churn[1]: release-arm64

Postgres 14

• test_ondemand_wal_download_in_replication_slot_funcs: release-arm64
• test_subscriber_restart: release-arm64

Code coverage* (full report)

• functions: 32.1% (7447 of 23213 functions)
• lines: 49.9% (59953 of 120174 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
93b4f50 at 2024-09-23T18:52:41.935Z :recycle:}