Export Prometheus metrics for ResourceSet APIs

[stefanprodan]

This PR implements Prometheus instrumentation for the ResourceSet controllers.

ResourceSet Metrics

Metrics:

flux_resourceset_info{uid, kind, name, exported_namespace, ready, suspended, revision}

Labels:

• uid: The Kubernetes unique identifier of the resource.
• kind: The kind of the resource (e.g. ResourceSet).
• name: The name of the resource (e.g. podinfo).
• exported_namespace: The namespace where the resource is deployed (e.g. apps).
• ready: The readiness status of the resource (e.g. True, False or Unkown).
• reason: The reason for the readiness status (e.g. ReconciliationSucceeded, BuildFailed, HealthCheckFailed, etc.).
• suspended: The suspended status of the resource (e.g. True or False).
• revision: The revision last applied on the cluster (e.g. sha256:75aa209c6a...).

ResourceSetInputProvider Metrics

Metrics:

flux_resourcesetinputprovider_info{uid, kind, name, exported_namespace, ready, suspended, url}

Labels:

• uid: The Kubernetes unique identifier of the resource.
• kind: The kind of the resource (e.g. ResourceSetInputProvider).
• name: The name of the resource (e.g. podinfo-prs).
• exported_namespace: The namespace where the resource is deployed (e.g. podinfo-review).
• ready: The readiness status of the resource (e.g. True, False or Unkown).
• reason: The reason for the readiness status (e.g. ReconciliationSucceeded or ReconciliationFailed).
• suspended: The suspended status of the resource (e.g. True or False).
• url: The provider address (e.g. https://github.com/stefanprodan/podinfo).