Implement Prometheus instrumentation for Flux resources

[stefanprodan]

This PR improves the observability capabilities of the operator by exporting Prometheus metrics for Flux instance and Flux custom resources. These metrics can be used to monitor both the Flux installation and the Flux-managed deployments, giving users a holistic status report of the cluster state reconciliation.

In addition, this PR makes the operator conform with OpenShift/OLM “Deep Insights” requirements.

Closes: #53

Flux Instance Metrics

The Flux Operator exports metrics for the FluxInstance resource.
These metrics are refreshed every time the operator reconciles the instance.

Metrics:

flux_instance_info{uid, kind, name, exported_namespace, ready, suspended, registry, revision}

Labels:

• uid: The Kubernetes unique identifier of the resource.
• kind: The kind of the resource (e.g. FluxInstance).
• name: The name of the resource (e.g. flux).
• exported_namespace: The namespace where the resource is deployed (e.g. flux-system).
• ready: The readiness status of the resource (e.g. True, False or Unkown).
• reason: The reason for the readiness status (e.g. Progressing, BuildFailed, HealthCheckFailed, etc.).
• suspended: The suspended status of the resource (e.g. True or False).
• registry: The container registry used by the instance (e.g. ghcr.io/fluxcd).
• revision: The Flux revision installed by the instance (e.g. v2.3.0@sha256:75aa209c6a...).

Flux Resource Metrics

The Flux Operator exports metrics for all Flux resources found in the cluster.
These metrics are refreshed at the same time with the update of the FluxReport.

Metrics:

flux_resource_info{uid, kind, name, exported_namespace, ready, suspended, ...}

Common labels:

• uid: The Kubernetes unique identifier of the resource.
• kind: The kind of the resource (e.g. GitRepository, Kustomization, etc.).
• name: The name of the resource (e.g. flux-system).
• exported_namespace: The namespace of the resource (e.g. flux-system).
• ready: The readiness status of the resource (e.g. True, False or Unkown).
• reason: The reason for the readiness status (e.g. Progressing, BuildFailed, HealthCheckFailed, etc.).
• suspended: The suspended status of the resource (e.g. True or False).

Specific labels per resource kind:

Resource Kind	Labels
Kustomization	revision, source_name, path
GitRepository	revision, url, ref
OCIRepository	revision, url, ref
Bucket	revision, url, ref
HelmRelease	revision, source_name
HelmChart	revision, source_name
HelmRepository	revision, url
Receiver	url
ImageRepository	url
ImagePolicy	source_name
ImageUpdateAutomation	source_name

[souleb]

LGTM!