David Montoya

this is a last mile doc improvement for https://github.com/docker/login-action/issues/640. it clarifies how to use direct identity federation in GAR without a service account using `google-github-actions/auth@v2`.

for `modules/mig`, allow setting [filter](https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_region_autoscaler#filter-2) on the autoscaling metric in order to scale a group with a custom metric selector. this would be useful for module [terraform-google-modules/terraform-google-github-actions-runners](https://github.com/terraform-google-modules/terraform-google-github-actions-runners/blob/196bf3c54b0283f9862fb3fade9b68f4d911fe3e/modules/gh-runner-mig-vm/main.tf#L155-L168) to allow scaling...