Network Observability: Custom metrics API GA

Follow-up on NETOBSERV-1341, which was a dev-preview: make this new custom metrics API GA.

• Address any feedback / improvement requested
• QE work
• Documentation work

Why is this important?

NetObserv is able to generate many different metrics out of flow logs, but this is currently a locked-down API, providing just an opinionated set of metrics. By opening this API, customers would be able to get metrics tailored for their needs.

Metrics are becoming more and more a key part of NetObserv for at least two reasons:

• the ability to create alerts
• the "lokiless" use case gaining traction, which still produces metrics

Providing the ability for customer to define their own metrics with a CRD-based API will allow them to get exactly what they want.

Moreover, on the engineering side, this is offloading some maintenance aspects so we don't need to maintain a huge list of available metrics.