Add new Workbench prometheus endpoint

[tnederlof]

Workbench 2024.04.0 released a new Prometheus endpoint as a preview feature: https://docs.posit.co/ide/server-pro/auditing_and_monitoring/prometheus_metrics.html#prometheus-metrics

This would be especially helpful to have in the helm charts as many using Workbench in K8s use Prometheus for metrics. Since this is preview probably still worth keeping the graphite exporter as the default but allow an easy way to opt into the native Prometheus endpoint instead.