Export db size, deadlocks and changed row metrics

[zaynetro]

Problem

We want to report metrics for the oldest user database.

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]

2490 tests run: 2368 passed, 0 failed, 122 skipped (full report)

---

Flaky tests (3)

Postgres 16

• test_compute_pageserver_connection_stress: release
• test_secondary_downloads: debug

Postgres 14

• test_compute_pageserver_connection_stress: debug

Code coverage* (full report)

• functions: 28.8% (7026 of 24429 functions)
• lines: 47.6% (43439 of 91322 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
99891ad at 2024-03-08T13:54:23.271Z :recycle:}

[vadim2404]

if you cherry-pick your commit to this PR (#7081) it will be released this week