chore: ASM module - Add a note about the mesh_id label on the GKE cluster (terraform-google-modules#1284)

* Add a note about the `mesh_id` label on the GKE cluster

* Update modules/asm/README.md

Co-authored-by: Bharath KKB <[email protected]>

* Link to a doc talking about the `mesh_id` label on the GKE cluster

Co-authored-by: Bharath KKB <[email protected]>

Author: mathieu-benoit

modules/asm/README.md

@@ -33,6 +33,15 @@ module "asm" {
 }
 ```
 
+Note that the [`mesh_id` label on the cluster](https://cloud.google.com/service-mesh/docs/managed/auto-control-plane-with-fleet#apply_the_mesh_id_label) is required for metrics to get displayed on the Anthos Service Mesh pages in the Cloud console (Topology, etc.). Illustrated with the full example mentioned above, here is an example of what your cluster should have:
+```tf
+module "gke" {
+...
+  cluster_resource_labels = { "mesh_id" : "proj-${data.google_project.project.number}" }
+...
+}
+```
+
 To deploy this config:
 
 1. Run `terraform apply`