Fixes: coder/internal#767
Adds two new Prometheus metrics for license health monitoring:
- `coderd_license_warnings` - count of active license warnings
- `coderd_license_errors` - count of active license errors
Metrics endpoint after startup of a deployment with license enabled:
```
...
# HELP coderd_license_errors The number of active license errors.
# TYPE coderd_license_errors gauge
coderd_license_errors 0
...
# HELP coderd_license_warnings The number of active license warnings.
# TYPE coderd_license_warnings gauge
coderd_license_warnings 0
...
```