feat: Add detailed Prometheus metrics

[ajaysundark]

The controller has a metrics endpoint (currently disabled by default), but we're not exporting any custom metrics yet. For production deployments, admins need visibility into the controller.

Some of the interesting metrics can be -

1. How many rules are active?
2. How many nodes getting stuck?
3. How long do evaluations take?
4. Taints added / removed
5. Evaluation failures
6. Bootstrap completion metrics
7. Error / operational failure metrics