monitoring + auth: disable Grafana login when using Pomerium

[ralgozino]

When using Pomerium to protect the ingresses with (OIDC) login, after logging in, when you visit Grafana it still shows the option to login (to Grafana's login system), confusing the user that they may not be logged in.

When auth with Pomerium is enabled, Grafana's login option should be disabled:
https://grafana.com/docs/grafana/latest/setup-grafana/configure-security/configure-authentication/grafana/

or, even better, integrate the auth with Pomerium:
https://grafana.com/docs/grafana/latest/setup-grafana/configure-security/configure-authentication/auth-proxy/
(to be checked what happens with the permissions)

See also Pomerium's guide:
https://www.pomerium.com/docs/guides/grafana

We should check the rest of the UIs we expose for the core modules for the same problem.