chore(pageserver): reduce logging related to image layers

[skyzh]

Problem

• Reduce the logging level for create image layers of metadata keys. (question: is it possible to adjust logging levels at runtime?)
• Do a info logging of image layers only after the layer is created. Now there are a lot of cases where we create the image layer writer but then discarding that image layer because it does not contain any key. Therefore, I changed the new image layer logging to trace, and create image layer logging to info.

Summary of changes

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]

3108 tests run: 2981 passed, 0 failed, 127 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_crafted_wal_end[wal_record_crossing_segment_followed_by_small_one]: debug
• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 31.4% (6450 of 20545 functions)
• lines: 48.3% (49871 of 103281 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e14adaa at 2024-05-23T15:39:37.710Z :recycle:}