Expose metrics for rig-platform and rig-operator

[andersjohnsen]

It should be possible to get basic request and system metrics for the services.

• [x] rig-operator #404 #406
• [ ] Add prometheus endpoint for rig-platform
• [ ] Add ServiceMonitor in rig-platform chart
• [ ] Add documentation

[blacksails]

rig-operator has metrics exposed at :8080/metrics. We should probably add a section in the docs about this.

[blacksails]

We could also add an option for creating a ServiceMonitor resource in the helm chart. Tracking issue: #404