add config option to optionally create `ServiceMonitor` resource

[kadel]

• ServiceMonitor instructs OpenShift Monitoring stack (Prometheus Operator) to scrape metrics (https://github.com/janus-idp/backstage-showcase/pull/469) from Backstage.

epic: https://github.com/janus-idp/backstage-showcase/issues/229

[tumido]

We support this via: upstream.metrics.serviceMonitor.enabled=true

https://github.com/backstage/charts/blob/main/charts/backstage/templates/servicemonitor.yaml