Description of problem:
Thanos Ruler sends alerts to the Alertmanager service (alertmanager-main.openshift-monitoring.svc) instead of all Alertmanager pods.
This might that each Alertmanager will get an incomplete view of the alerts depending which .

Version-Release number of selected component (if applicable):
4.5

How reproducible:
Always

Steps to Reproduce:
1. Enable user workload monitoring.
2. Create a user alert that always fires.
***
apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  name: test
  namespace: default
spec:
  groups:
  - name: Test rules
    rules:
    - alert: Drill
      expr: vector(1)
      labels:
        severity: warning
***

3.Query each Alertmanager for the list of active alerts

for i in {0,1,2}; do echo "alertmanager-main-$i"; oc exec -n openshift-monitoring -t  alertmanager-main-$i -c alertmanager --  curl -s http://localhost:9093/api/v2/alerts | jq -r '.[].labels.alertname'; done


Actual results:

alertmanager-main-0
AlertmanagerReceiversNotConfigured
Watchdog
Drill
alertmanager-main-1
AlertmanagerReceiversNotConfigured
Watchdog
Drill
alertmanager-main-2
AlertmanagerReceiversNotConfigured
Watchdog

Expected results:
The "Drill" alert should be present for every Alertmanager.

Additional info:
Right now Thanos Ruler is configured with to with "alertmanager-main.openshift-monitoring.svc:9094". It needs to be "dnssrv+_web._tcp.alertmanager-operated.openshift-monitoring.svc" instead. But for this to work, a first fix is needed in prometheus-operator.