Prometheus UI is broken after adding authentication

After implementing #242, Prometheus UI is broken. It doesn't pass any basicAuth headers to GMP. I solved it by adding some browser extension. Here's how:

1. Configure Prometheus UI to use AUTH_USERNAME and AUTH_PASSWORD
2. Run Prometheus UI:
``` kubectl port-forward --namespace prometheus svc/infra-monitoring-prometheus-frontend-production 8080:9090```
3. Open the browser at http://localhost:8080, and make sure it's failing to load metrics

But Grafana access works fine.

I found a way to overcome it:
1. Install [Modify Header Value](https://mybrowseraddon.com/modify-header-value.html) addon to your browser.
6. In addon, setup basic auth header with the values from your prometheus chart.
7. Make sure your prometheus dashboard is now working (i.e. no erros, metrics can be searched, etc.).