expose CEL execution prometheus metrics

[a-hilaly]

Feature Description

Currently kro lacks observability around CEL expression execution. we need to add prometheus metrics similar to the Kubernetes API server to track CEL evaluation performance and failures. These metrics will help operators monitor the health and performance of ResourceGroups/I reconcilers.

We should follow the pattern from:
https://github.com/kubernetes/apiserver/blob/2241ea00526fdd746871375dfca871627f6a511a/pkg/cel/metrics/metrics.go#L32-L62

[harshithsaiv]

Hi @a-hilaly !

I would love to contribute to this. Can I give this a try? If possible, could you assign it to me?

Thanks!

[a-hilaly]

@a-hilaly Absolutely, thank you!