gnuruwi, The "Metrics based alert rules" can be used to target all the resources in a single subscription, even though when residing in different resource groups in this subscription. Using azurerm_monitor_metric_alert provider, you may use the combination of the following arguments to achieve this:
Scope - Set this to Subscription resource id (for example - /subscriptions/<SUBSCRIPTION_ID>
)
It seems that currently you have used Scope to target a single resource group, therefore the rule only gets applied to VM in that resource group. If the target is done at Subscription level, then all the resource groups inside this subscription and consequently all the VMs in these resource groups becomes the target.
Furthermore, criteria should be used to refine the target. For example, if the metric based alert rule is for VM, the criteria should specify the criteria
.target_resource_type
as Microsoft.Compute/virtualMachines
This will edit this metric rule to target all VMs in this specific scope (which the subscription in this case).
Regarding the limitations, the limitations as available for Subscription Management and Monitoring resources also applies when the resources are deployed through terraform:
Hope this helps. Please let me know if you have any questions.