Fix collecting metrics (#4822)

# What this PR does
Reverts the accidental removal of the ApplicationMetricsCollector from
the metric register

## Which issue(s) this PR closes

Related to [issue link here]

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## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/metrics_exporter/metrics_collectors.py

@@ -232,3 +232,6 @@ def recalculate_cache_for_missing_org_ids(self, org_ids: set[int], missing_org_i
                 recalculate_orgs.append({"organization_id": org_id, "force": force_task})
         if recalculate_orgs:
             start_calculate_and_cache_metrics.apply_async((recalculate_orgs,))
+
+
+application_metrics_registry.register(ApplicationMetricsCollector())