1. Proposed title of this feature request
Separate alerts by source (cluster and customer).
2. What is the nature and description of the request?
Enforce a specific label depending on where the alert is coming from, without modifying the existing `prometheus` label.
3. Why does the customer need this? (List the business requirements here)
The best way is to quote the customer:

What we functionally want is a clear separation between clusteralerts and customer alerts (generated by customers in their own namespaces) so they can be routed easily and the alertmanager configuration isn't prone to human errors . One option could be 2 separate alertmanager instances maybe, or a clear distinct label that is automatically/forcibly added to all user-created alerts or prometeusrules so we can filter based on that. As one of my collegueas already mentioned we obviously don't want to miss or discard some cluster alerts, for example by making a configuration error in the alertmanager config.

4. List any affected packages or components.
Prometheus, alertmanager, thanos