SELF_HOSTED_SETUP: Document prometheus metrics port

there's no /metrics endpoint or port given in the documentation for amazon-eks-pod-identity-webhook
Since this sits in the path for pod creation for performance metrics are required.
Documentation for auto-scaling the controller is required.
Some visibility into usage of role to deploy mapping for grafana dashboard is required

Start by locating the self-hosted setup documentation and any existing Prometheus or Grafana references in the repository. Determine what metrics endpoint and port, controller auto-scaling guidance, and role-to-deployment usage visibility are available. Done means the documentation clearly covers all three requested areas.

Written by the indexing model from the issue text.