Add relation and expose metrics from jupyter-ui

### Context

Implement metrics-endpoint for the jupyter-ui, after [#7633](https://github.com/kubeflow/kubeflow/issues/7633) is fixed in upstream and changes are included in release (e.g. 1.10).

We need to add relation using `prometheus_scrape` interface and `grafana_dashboard` interface if any dashboard is provided.

### What needs to get done

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

### Definition of Done

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

First confirm that upstream kubeflow/kubeflow#7633 is fixed and available in the target release, then inspect the jupyter-ui charm's ServicePort and COS integration points. Implement and verify the metrics-endpoint, add grafana-dashboard support if a dashboard exists, and run the COS integration tests; done means both endpoints are exposed and the integration is tested.

Written by the indexing model from the issue text.