Add ServiceMonitor support for the login UI metrics

[mridang]

Preflight Checklist

• I could not find a solution in the existing issues, docs, nor discussions
• I have joined the ZITADEL chat

Describe your problem

The chart only exposes a ServiceMonitor for the main deployment. The login deployment does not emit or scrape metrics via a ServiceMonitor, so login-specific metrics cannot be collected by Prometheus when enabled.

Describe your ideal solution

As a chart maintainer, I want an optional ServiceMonitor for the login deployment (when login.enabled and login metrics are exposed), with values to control enablement, labels/namespace, scrape interval/timeout, and TLS/headers as needed.

Files to remove/modify:

• Add a ServiceMonitor template for the login service (analogous to templates/servicemonitor.yaml)
• Add values under login.metrics (e.g., enabled, serviceMonitor.*) and wire them in docs/examples

Version

None

App version

5.x

Additional Context

This will align with the upcoming feature where the login container exposes metrics, allowing Prometheus to scrape them when enabled.