helm: Enrich serviceMonitor properties

[jkroepke]

Required for custom relabeling actions or istio deployments.

[pierluigilenoci]

@jkroepke, could you please add the ArtifactHub annotation here?
https://github.com/oauth2-proxy/manifests/blob/main/helm/oauth2-proxy/Chart.yaml#L35-L41

There are some missing default values here:
https://github.com/oauth2-proxy/manifests/blob/main/helm/oauth2-proxy/values.yaml#L362-L374

Specifically:

scheme
bearerTokenFile
tlsConfig

And an unused one, targetLabels.

So please fix your PR.

[pierluigilenoci]

Documentation should also be updated with the changes.
https://github.com/oauth2-proxy/manifests/blob/main/helm/oauth2-proxy/README.md?plain=1#L204-L209

I apologize for the fragmented review.
To speed up, I'm making suggestions while busy with other tasks.
I hope I can be of help anyway.

[jkroepke]

I hope I hit all your points

[pierluigilenoci]

I hope I hit all your points

Unfortunately not yet.

[jkroepke]

After #171, serviceMonitor is valid now.

please review.