vm-monitor: collect lfc stats from vm-monitor

[skyzh]

Problem

We do not have LFC metrics for autoscaling.

Summary of changes

This pull request adds a prometheus metrics endpoint for all LFC metrics. The LFC metrics are refreshed every 5 seconds.

Alternative: we can push the data over websocket, or collect the data when a client accesses /metrics instead of for a given interval.

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]

2754 tests run: 2630 passed, 0 failed, 124 skipped (full report)

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Code coverage* (full report)

• functions: 28.0% (6404 of 22875 functions)
• lines: 46.8% (45074 of 96238 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
393f323 at 2024-04-05T16:31:17.470Z :recycle:}

[skyzh]

Need to set up vm-monitor to test the change, waiting CI for building the image

[sharnoff]

I think adding it to vm-monitor is probably not the way we want to go? — we already have LFC metrics available from sql-exporter. vm-monitor is running only when the autoscaler-agent is already successfully connected to it; it is more fragile than sql-exporter for that reason.
And either way, we need to add support to the autoscaler-agent to fetch LFC metrics, so we might as well get them from sql-exporter.

Maybe I'm missing something - happy to discuss, lmk.