Replace broker duration logs with metrics

[petuhovskiy]

Summary of changes

I've added logs for broker push duration after every iteration in #4142. This log has not found any real issues, so we can replace it with metrics, to slightly reduce log volume.

LogQL query found that pushes longer that 500ms happened only 90 times for the last month. https://neonprod.grafana.net/goto/KTNj9UwVg?orgId=1

{unit="safekeeper.service"} |= "timeline updates to broker in" | regexp "to broker in (?P<duration>.*)" | duration > 500ms

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]

1004 tests run: 963 passed, 0 failed, 41 skipped (full report)

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_{The comment gets automatically updated with the latest test results
1115897 at 2023-05-30T11:46:31.434Z :recycle:}