Integrated Alerting: Filtered out alerts are shown in the UI as firing

User impact:
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;

Actual result:

If an alert rule contains a filter that does not match the firing label, it is still shown in the PMM UI as "firing"

Expected result:

It either should not be shown at all or shown with some other status (maybe a new status should be created for that case).

Details:

In API terms, alerts with the only receiver being "empty" should be shown differently.