Add serviceMonitor

[Sea-you]

Add ServiceMonitor object to easily set up metrics scraping with Prometheus Operator.

As you can see, the change is backward incompatible as metricsEnabled has been renamed to metrics.enabled

[bootc]

I'd like to have this but:

• It feels like it would be easy to avoid the backwards-incompatible change. Perhaps something like this (not tested)?

METRICS_ENABLED: {{ .Values.metricsEnabled | default .Values.metrics.enabled | toJson }}

• The scrape interval and timeout feel like they should go under the metrics.serviceMonitor key, as they're a Prometheus property rather than the metrics gathering system in NetBox.

What do you think?

[bootc]

@Sea-you did you have a chance to consider my suggestions?