neonvm: add LFC approximate working set size to metrics

[skyzh]

Problem

ref neondatabase/autoscaling#878
ref neondatabase/autoscaling#872

Summary of changes

Add approximate_working_set_size to sql exporter so that autoscaling can use it in the future.

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

[skyzh]

do not merge until #7231 gets merged

[Bodobolero]

We have clarified in slack that we need to make sure that neon extension in compute VM is upgraded to latest version BEFORE this PR is merged and deployed in prod.
If they are deployed together, we will have some ephemeral errors in compute log at the first compute start after new image is in prod, but after that no more, which is fine.

[github-actions]

2730 tests run: 2592 passed, 0 failed, 138 skipped (full report)

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Code coverage* (full report)

• functions: 28.2% (6298 of 22348 functions)
• lines: 47.0% (44250 of 94192 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
06e37f2 at 2024-03-26T17:12:01.659Z :recycle:}