I have many resources groups. I have assigned special tags on these groups. I would like to have cost report like how many cost was spent in some month for all resources located in some groups with specific tags, e.g system=app1;workload = prod. So using tags like filter.
As you probably know, tags applied on Resource Groups are not automatically propagated down (nested) to underlying resources. In addition, if you access your Azure spend data using Cost Management APIs, they support tags (announced in March).
The problem is the fact that you need to implement a mechanism for applying tags (those specific to cost management) from RGs to resources. One way is utilizing Azure Automation that will execute a runbook (schedule-based execution) that can do it for you.