tf-variable-demo/modules/compute_engine_instance_group/main.tf

resource "google_compute_instance_group_manager" "instance_group" {
  name        = "instance-group"
  project     = var.project_id
  region      = var.region
  base_instance_name = "instance"
  instance_template = var.template_self_link
  target_size  = 2
}