Add EC2 StatusCheckFailed_AttachedEBS metric

This tool supports `StatusCheckFailed_System` and `StatusCheckFailed_Instance` metrics, but is missing `StatusCheckFailed_AttachedEBS` which is equally important for EC2 health monitoring during Countdown scenarios.

This metric monitoring is recommended by AWS best practices documentation:
- EC2 - Incident Detection and Response Alarming Best Practices | AWS re:Post https://repost.aws/articles/ARoLaAGDjcTf-FmETSfqF7GA

Start by locating the existing StatusCheckFailed_System and StatusCheckFailed_Instance metric definitions and the code that includes them in generated EC2 dashboards. Add the parallel StatusCheckFailed_AttachedEBS metric, then verify the generated dashboard contains it alongside the existing status-check metrics.

Written by the indexing model from the issue text.