Move load_timeline_metadata into separate function

[arpad-m]

Moves the per-timeline code to load timeline metadata into a new dedicated function called load_timeline_metadata. The old load_timeline_metadata becomes load_timelines_metadata.

Split out of #8907

Part of #8088

[koivunej]

per the threads.

[koivunej]

It should work now just fine. It took me a while to understand why did you make it an fn instead of async fn.

[github-actions]

5020 tests run: 4856 passed, 0 failed, 164 skipped (full report)

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Flaky tests (6)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64, release-x86-64
• test_secondary_background_downloads: release-x86-64
• test_subscriber_restart: release-x86-64

Postgres 15

• test_ondemand_wal_download_in_replication_slot_funcs: release-arm64
• test_subscriber_restart: release-x86-64

Code coverage* (full report)

• functions: 31.9% (7432 of 23316 functions)
• lines: 49.9% (59921 of 120123 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
89872e7 at 2024-09-21T12:47:59.449Z :recycle:}

[arpad-m]

test failure is legit:

thread 'tenant::tests::timeline_load_with_ancestor' panicked at pageserver/src/tenant.rs:1347:9:
missing extractors: [ConstExtractor { field_name: "timeline_id" }]

made commit to fix it.

[koivunej]

move assert into future

Aah yes, makes sense.

[koivunej]

Failures on Postgres 17

• test_subscriber_restart: release-x86-64

I don't understand this.

Failures on Postgres 15

• test_subscriber_restart: release-x86-64

This is more normal.