pageserver: tweak logging of "became visible" for layers

[jcsp]

Problem

Recent change to avoid the "became visible" log messages from certain tasks missed a task: the logical size calculation that happens as a child of synthetic size calculation.

Related: #9058

Summary of changes

• Add OnDemandLogicalSize to the list of permitted tasks for reads making a covered layer visible
• Tweak the log message to use layer name instead of key: this is more terse, and easier to use when debugging, as one can search for it elsewhere to see when the layer was written/downloaded etc.

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]

5022 tests run: 4864 passed, 0 failed, 158 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_timeline_archive[4]: debug-x86-64

Code coverage* (full report)

• functions: 31.3% (7486 of 23904 functions)
• lines: 49.6% (60104 of 121238 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
38fcafe at 2024-10-01T15:02:21.502Z :recycle:}