Managed Instance Group Discovery

The extension discovers zonal and regional Managed Instance Groups (MIGs) across accessible projects. Each MIG is exposed as a target with attributes for its scope (zonal / regional), target size, instance template, autoscaling configuration, health-check policy, and base instance name.

Note: MIGs backing GKE node pools are also discovered here, but are usually attacked through the GKE node pool target instead to keep pool ownership semantics intact.