Add metric for replicas_desired

[dudadornelles]

Summary

Add a new metric rollout_info_replicas_desired to represent the number of replicas you ultimately desire your rollout to have

Use Cases

We are moving from Deployments to Rollouts and having problems migrating our datadog monitors to use the available rollout metrics. We have monitoring around number of replicas unavailable to help us identify degradation of service, but in argo-rollouts, the rollout_info_replicas_unavailable is the number of available rollout replicas which I understand are considered "unavailable" because they aren't live yet?

Either ways, with a replicas_desired metric (which would be simply Rollout.Spec.Replicas) I could calculate the number of really unavailable replicas with desired - available

As reference this is implemented in kube-state-metrics https://github.com/kubernetes/kube-state-metrics/blob/392f9249f808041612a8d3c5348fc48b64cece74/internal/store/deployment.go#L159

Happy to open a PR on this if there is interest.

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