Resource Groups: Emit metrics that could be used by external schedulers

Determine what kind of metrics should be emitted so that external resource schedulers (e.g., [HPA](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/), [KEDA](https://keda.sh/), etc.) can add or remove compute resources based on demand. For example, external compactor scaling based on compaction queues. Or, scaling the number of ScanServers based on current ScanServer load and number of clients performing scans.

Start by reviewing the issue's examples involving external compaction scaling, ScanServer load, scan clients, HPA, and KEDA. Determine which demand signals external schedulers would need and how completion would be judged; the payload names no source files, tests, or entry points.

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