feat(server): add a gauge tracking feature flag rollout across workspaces (#24685)

## What

Adds `twenty_feature_flag_enabled_workspaces`, an OpenTelemetry
observable gauge (exported to Prometheus) labelled by `flag_key`,
reporting how many workspaces have each feature flag enabled.

This gives a metric-based view of a per-workspace feature flag rollout
(e.g. the ORM v2 read path) so progress can be tracked on a dashboard
instead of ad-hoc DB queries against `core."featureFlag"`.

## How

- New `FeatureFlagMetricsService` (`OnModuleInit`) follows the existing
`DatabaseGaugeService` pattern: it registers a
`createMultiObservableGauge` whose callback runs `SELECT key, COUNT(*)
... WHERE value = true GROUP BY key` against the core datasource and
emits one observation per flag with `{ flag_key }` as the attribute.
- Wired into `FeatureFlagModule` (imports `MetricsModule`, provides the
new service).
- The query result is cached (`cacheValue: true`, 60s TTL in
`MetricsService`), so each pod runs at most one small count query per
minute. Cardinality is one series per feature flag (low).

## Notes

- Every server pod reports the same DB-derived count, so dashboards
should aggregate across replicas with `max by (flag_key)`.
- The companion Grafana dashboard lives in twenty-infra (`Twenty /
Feature Flag Release`).
- No spec added, matching the existing gauge/metric services
(`DatabaseGaugeService`, `EventLoopMetricsService`), which have none.

## Test plan

- [ ] Typecheck / lint pass (CI).
- [ ] After deploy, confirm
`twenty_feature_flag_enabled_workspaces{flag_key="IS_ORM_V2_READ_PATH_ENABLED"}`
appears in Prometheus and matches the row count in `core."featureFlag"`.


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