As we know, we could use the Azure portal, PowerShell or the Command Line Interface etc to create alerts rule to monitor Azure resource metrics, events or logs. And we need to specify the Resource ID for the resource that we want to set and alert for, and different resources with different Resource ID, so we could not create a global alerts rule for monitoring all of our web apps and slots.
Resource ID may look like this.
Web app:
"id": "/subscriptions/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/resourceGroups/YourResourceGroupName/providers/Microsoft.Web/sites/websitename"
Web app slot:
"id": "/subscriptions/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/resourceGroups/YourResourceGroupName/providers/Microsoft.Web/sites/websitename/slots/slotname"