Enable jupyter-ui metrics collector, grafana dashboard, and alert rules

### Why it needs to get done

This task aims at getting this charm integrated with COS. Recent changes in upstream now allow to enable the metrics exporter of this component, meaning it can now be integrated with the Observability solutions.

### What needs to get done

1. Expose port for metrics as ServicePort
2. Implement metrics-endpoint endpoint
3. Implement grafana-dashboard endpoint
4. Find and add alert rules for metrics from upstream
5. Find and add Grafana dashboards for metrics from upstream
6. Add basic alert rules

### When is the task considered done

1. kubeflow-volumes has `metrics-endpoint` and `grafana-dashboard` endpoints
2. Integration with cos is tested

Start by locating the charm's ServicePort configuration and existing endpoint implementations. Review upstream metrics, alert rules, and Grafana dashboards, then verify that kubeflow-volumes exposes metrics-endpoint and grafana-dashboard and that COS integration is covered by tests.

Written by the indexing model from the issue text.