feat(compute): Introduce new compute_subscriptions_count metric

[ololobus]

Problem

We need some metric to sneak peek into how many people use inbound logical replication (Neon is subscriber).

Summary of changes

This commit adds a new metric compute_subscriptions_count, which is number of subscriptions grouped by enabled/disabled state.

Resolves: neondatabase/cloud#16146

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]

Have you tested this against all Postgres versions? Note that we don't have regression tests for sqlexporter unfortunately #8569

[ololobus]

I tested in PG14-16, yes, but only in 16 I had at least 1 subscription. I still assume that if the schema didn't change, it's OK

[github-actions]

3827 tests run: 3721 passed, 0 failed, 106 skipped (full report)

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Flaky tests (5)

Postgres 15

• test_scrubber_physical_gc[None]: release
• test_scrubber_scan_pageserver_metadata[None]: release
• test_scrubber_physical_gc_ancestors_split: release
• test_creating_tenant_conf_after_attach: release
• test_tenant_delete_smoke: release

Code coverage* (full report)

• functions: 32.4% (7245 of 22346 functions)
• lines: 50.4% (58497 of 116048 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
01f7e30 at 2024-08-22T14:56:06.649Z :recycle:}