The user can not filter alerts.
Steps to reproduce:
1. Navigate to IA and create two similar alert rules but with different filters (e.g. one that matches service_name and the second that doesn't match);
2. Wait until the rule is executed and verify the Alerts tab;
If an alert rule contains a filter that does not match the firing label, it is still shown in the PMM UI as "firing"
It either should not be shown at all or shown with some other status (maybe a new status should be created for that case).
In API terms, alerts with the only receiver being "empty" should be shown differently.