## Summary
- Adds `UpgradeGaugeService` that exposes three observable Prometheus
gauges based on the recently merged upgrade status service:
- `twenty_upgrade_instance_health` — 1 (up-to-date), 0 (behind), -1
(failed)
- `twenty_upgrade_workspaces_behind_total` — count of workspaces with
pending upgrade commands
- `twenty_upgrade_workspaces_failed_total` — count of workspaces with a
failed upgrade command
- Follows the existing gauge pattern (`WorkspaceGaugeService`,
`BillingGaugeService`, `DatabaseGaugeService`)
### Caching & QPS design
Prometheus scrapes every **15s** via `ServiceMonitor`. Each gauge uses
the `MetricsService.createObservableGauge({ cacheValue: true })` pattern
which caches the value in Redis for **60 seconds**. Under that,
`UpgradeStatusService.getInstanceAndAllWorkspacesStatus()` uses
`UpgradeStatusCacheService` with a **1-hour TTL** in Redis.
Result: at most 1 DB query per hour regardless of scrape frequency.
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Co-authored-by: Cursor <cursoragent@cursor.com>
