feat: expose custom metrics for tenants status

[gernest]

closes #451

This commit adds pkg/stats that defines prometheus counters for active and cordoned tenants.

These metrics are exposed to controller-runtime. So they are visible on the controller metrics endpoint.

This was tested manually , after creating a new tenant the metrics showed up on the metrics service endpoint

# HELP capsule_tenants_status_active_count Total number of active tenants
# TYPE capsule_tenants_status_active_count counter
capsule_tenants_status_active_count 1

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[bsctl]

@prometherion is this something we can merge in master?

[adberger]

This is a good start :)
@prometherion any updates on this?

[MaxFedotov]

what do you think about using https://github.com/kubernetes/kube-state-metrics/blob/main/docs/customresourcestate-metrics.md? It allows to add custom metrics based on fields in CRD without a need to modify code

[adberger]

what do you think about using https://github.com/kubernetes/kube-state-metrics/blob/main/docs/customresourcestate-metrics.md? It allows to add custom metrics based on fields in CRD without a need to modify code

Solved according to your suggestion, made an example in the referenced issue (https://github.com/projectcapsule/capsule/issues/451).

[prometherion]

Closing in favour of https://github.com/projectcapsule/capsule/issues/451#issuecomment-1774537482.