Support reading external metrics to assess Service readiness

[infbase]

There are multiple scenarios where it would be useful to look at external metrics to assess service readiness, in addition to the current logic. For instance, by looking at nginx ingress controller metrics, scenarios such as abnormal performance or request count could be identified and the service determined to be unready.

A way to support this would be to add the ability to query Prometheus metrics, for instance somewhat like KEDA's Prometheus scaler does. When designing this implementation, it should be considered if support for Prometheus queries should be directly baked into k8gb or implemented thru an external call, such as:

• externally managed gRPC service, like KEDA's external scalers
• external call to promtool for instance

This implementation should not address single endpoint readiness. That should be done using standard K8s readiness probes even if calling external tools.

[abaguas]

KEDA exposes metrics from external sources via the Kubernetes metrics server as custom metrics: https://keda.sh/docs/2.16/operate/metrics-server/

To profit from this ecosystem we could have a metrics server client in K8GB. Then we would need to query only a single API and could benefit from countless integrations.
This repo may have a client that we can use: https://github.com/kubernetes/metrics. I will read up more in the coming days.