Use a Cloud Audit Logs trigger to invoke a Cloud Function when a Compute Engine VM is created. Check for missing labels and assign them if necessary.

Deploy resources with Terraform. Use the gcloud terraform vet command with a policy to ensure that every Compute Engine VM that is provisioned by Terraform has labels set.

Write a script to check all Compute Engine VMs for missing labels regularly by using Cloud Scheduler. Use the script to assign the labels.

Check all Compute Engine VMs for missing labels regularly. Use the console to assign the labels.