feat(stats): add banned gauge to pool metrics

[ChrisJr404]

Closes #1424. Adds a banned gauge to the per-pool OpenMetrics output so you can alert on a database being banned instead of inferring it from the socket counts, which don't reliably drop to zero for every kind of failure.

It reuses the same ban state that SHOW BANS reports (switched the metrics loop to pools_with_roles_and_bans), emitting one measurement per pool with the usual database/user/host/shard/role labels: 1 when the pool is banned, 0 when it's available. Added a render test next to the existing pool metric tests.

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[levkk]

Cool! Happy to merge if you sign the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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