storcon: track number of attached shards for each node

[VladLazar]

Problem

The storage controller does not track the number of shards attached to a given pageserver. This is a requirement for various scheduling operations (e.g. draining and filling will use this to figure out if the cluster is balanced)

Summary of Changes

Track the number of shards attached to each node.

Related #7387

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]

3198 tests run: 3056 passed, 0 failed, 142 skipped (full report)

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Code coverage* (full report)

• functions: 31.5% (6602 of 20948 functions)
• lines: 48.5% (51128 of 105464 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
3243802 at 2024-06-11T13:25:20.572Z :recycle:}

[arpad-m]

Approving this, skipping verification that the functions which call update_node_ref_counts actually are only called once (because even though update_node_ref_counts is not idempotent, neither is the functions it replaces).

We don't rely on demote_attached idempotency anywhere, right?

[VladLazar]

Approving this, skipping verification that the functions which call update_node_ref_counts actually are only called once (because even though update_node_ref_counts is not idempotent, neither is the functions it replaces).

We don't rely on demote_attached idempotency anywhere, right?

Right, demote_attached is not meant to be idempotent.