Document metrics_addr setting for Enterprise deployments

[ZPain8464]

When setting up metrics in Pomerium Enterprise, users must configure a metrics_addr setting in the Enterprise configuration file. This setting exposes the address where Prometheus can push internal console metrics to the Enterprise web console. This setting is specific to Enterprise deployments, but is not documented as a config setting on the Enterprise Configuration page.

We need to document this setting.

To avoid any confusion, metrics_addr is distinct from metrics_address. metrics_address must be defined as a bootstrap setting in the Core configuration file.

To successfully implement metrics in the Enterprise console, only metrics_addr is required, and it has a default IP and port (127.0.0.1:9092) if it's not defined. metrics_address is required for an external Prometheus instance in the Core configuration file.

As a follow-up item, it would be a good idea to add more clarification to metrics_address so users understand the relationship between these settings.