title: GCP subnetworks.listUsable does not return shared subnets
body: <p>I have a Shared VPC setup between 2 projects - Shared VPC Host Project P1 and Shared VPC Service Project P2. The host project P1 shares certain subnets with P2, which I can see on the console. I have Owner, Service Networking Admin and Compute Network User roles on the Host project and Owner and Service Networking Admin roles on the Service project.</p>
  
  <p>I am trying to call this API on service project P2</p>
  
  <p><a href="https://cloud.google.com/compute/docs/reference/rest/v1/subnetworks/listUsable" rel="nofollow noreferrer">https://cloud.google.com/compute/docs/reference/rest/v1/subnetworks/listUsable</a></p>
  
  <p>"Retrieves an aggregated list of all usable subnetworks in the project. The list contains all of the subnetworks in the project and the subnetworks that were shared by a Shared VPC host project"</p>
  
  <p>This API only returns all the subnets that were created in project P2 and none of the shared subnets. The gcloud equivalent "gcloud compute networks subnets list-usable" does the same thing.</p>
  
  <p>How do I get the list of subnets shared with a given project?</p>