chore(pageserver): temporary metrics on ingestion time

[skyzh]

Problem

As a follow-up on #7467, also measure the ingestion operation speed.

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]

2766 tests run: 2645 passed, 0 failed, 121 skipped (full report)

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Code coverage* (full report)

• functions: 28.1% (6483 of 23057 functions)
• lines: 46.9% (46063 of 98179 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
f023bf2 at 2024-04-25T16:46:51.675Z :recycle:}