Add labels support to flux_resource_info metrics

[jurim76]

We are using team-based labels in helmreleases and kustomizations and prometheus alert-manager sends alerts to teams. based on these labels.
Adding labels to flux_resource_info metric output will allow to send alerts about failed helmreleases and kustomizations to dedicated team.

[stefanprodan]

Adding all the Kubernetes labels as Prometheus labels by default would increase the cardinality by a lot and along with it, the cost of running Prometheus.

I think we should add this feature as an opt-in with a flag --metric-labels-allowlist=<regex>. We could maybe allow by default the standard Kubernetes labels as in:

--metric-labels-allowlist='^app\.kubernetes\.io\/(([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9])$'

We also have to prefix and sanitise the Kubernetes labels, for example app.kubernetes.io/version: "1.2.3" would become flux_resource_info{label_app_kubernetes_io_version="1.2.3"}. The sanitisation would have to replace any characters in the Kubernetes label key that are not valid in a Prometheus label key (like ., /, and -) with an underscore (_).