Report per pool average running requests metric

[ahg-g]

What would you like to be added:

An EPP metric that reports per pool average running requests similar to the one we have for queue size: https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/816899de9ea03c9c792aeb9100ffd750f2d24d24/pkg/epp/metrics/metrics.go#L296C16-L296C34

Why is this needed:
Observability

[ahg-g]

/assign @BenjaminBraunDev