fix: set prometheus authentication variable

[DnPlas]

This variable allows public access without authentication for prometheus metrics.

Part of canonical/bundle-kubeflow#564

Testing

Please refer to the steps to reproduce in this comment, just deploying this app. After deploying this app and cos-lite, relations and dependencies, and waiting a couple minutes (10 min) none of the alerts should fire (for this app only).

TODO

• [x] refactor integration test case test_prometheus_grafana_integration
• [x] create a backport PR for track/ckf-1.8 https://github.com/canonical/minio-operator/pull/158