fix: ResidentLayer::load_keys should not create INFO level span

[koivunej]

Since #6115 with more often used get_value_reconstruct_data and friends, we should not have needless INFO level span creation near hot paths. In our prod configuration, INFO spans are always created, but in practice, very rarely anything at INFO level is logged underneath. ResidentLayer::load_keys is only used during compaction so it is not that hot, but this aligns the access paths and their span usage.

PR changes the span level to debug to align with others, and adds the layer name to the error which was missing.

Split off from #7030.

[github-actions]

2706 tests run: 2576 passed, 0 failed, 130 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_vm_bit_clear_on_heap_lock: release

Code coverage* (full report)

• functions: 28.3% (7132 of 25174 functions)
• lines: 46.9% (43730 of 93326 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
1387f28 at 2024-03-19T13:49:33.610Z :recycle:}