Expose Prometheus metrics on rollouts

[dthomson25]

Argo Rollouts should expose Prometheus metrics in order to provide better overall insight into the rollouts in a cluster. Some initial metrics to include could be:

• Number of rollouts and which strategies they are using
• When a rollout starts progressing from a new pod spec
• When a rollout finishes progressing from a new pod spec
• Every time the rollout increments a step

As a reference implement, we can probably use argoproj/argo-workflows@b9cffe9 as a baseline

[dthomson25]

The metrics were improved with #61