TL;DR
Would be nice if we can add custom learned IP ranged in the peer section because it's something that can be added in the google_compute_router_peer resource, but not in this module.
Terraform Resources
https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_router_peer
https://registry.terraform.io/modules/terraform-google-modules/cloud-router/google/7.0.0/submodules/interconnect_attachment
Detailed design
Additional information
No response