https://cloud.google.com/compute/docs/instance-groups
https://cloud.google.com/load-balancing/docs/network/transition-to-backend-services#console
* In order to enable auto-healing, you need to group the instances into a managed instance group.
Managed instance groups (MIGs) maintain the high availability of your applications by proactively keeping your virtual machine (VM) instances available. An auto-healing policy on the MIG relies on an application-based health check to verify that an application is responding as expected. If the auto-healer determines that an application isnt responding, the managed instance group automatically recreates that instance.
It is important to use separate health checks for load balancing and for auto-healing. Health checks for load balancing can and should be more aggressive because these health checks determine whether an instance receives user traffic. You want to catch non-responsive instances quickly, so you can redirect traffic if necessary. In contrast, health checking for auto-healing causes Compute Engine to proactively replace failing instances, so this health check should be more conservative than a load balancing health check.

This command correctly specifies the duration that the signed url should be valid for by using the -d flag. The default is 1 hour so omitting the -d flag would have also resulted in the same outcome. Times may be specified with no suffix (default hours), or with s = seconds, m = minutes, h = hours, d = days. The max duration allowed is 7d.
Ref: https://cloud.google.com/storage/docs/gsutil/commands/signurl

https://cloud.google.com/monitoring/alerts#alerting-example