[Feature] Expose OIDC server status via monitoring

[viq]

Use case

This would allow easy checking of what headscale thinks of availability of the OIDC server, and to set up alerting based on such.

Description

Currently if the OIDC server is not available, headscale (if configured to do so) will switch to "standard" mode, and emit a message in logs. But if someone doesn't look at logs constantly, the only warning about this will be when registering a new machine instead of getting redirected to OIDC login, one will get the information about commands to run on server.

Headscale already has support for exposing some of internal information to prometheus. It would be great if this information was available there as well.

Contribution

• I can write the design doc for this feature
• I can contribute this feature

How can it be implemented?

Possibly metrics like headscale_oidc_configured or headscale_oidc_info with some information about the configured OIDC server, if any, and headscale_oidc_functional with 0 or 1 depending what the internal checks show.