Add openstack_web_proxy-backend haproxy backend to alert exceptions

This backend is used for frontend with redirect to SSL and never used.

Related-Prod: PROD-34125
Change-Id: Ib3c2f7273966db7b42f7a01c1c8aed192c9ab631
diff --git a/haproxy/meta/prometheus.yml b/haproxy/meta/prometheus.yml
index cd3e950..58e443b 100644
--- a/haproxy/meta/prometheus.yml
+++ b/haproxy/meta/prometheus.yml
@@ -61,8 +61,8 @@
         description: "{{ $value }} {{ $labels.proxy }} back ends (>= 50%) are down."
     HaproxyBackendOutage:
       if: >-
-        max(haproxy_active_servers{sv="BACKEND", proxy!="stats"}) by (proxy)
-        + max(haproxy_backup_servers{sv="BACKEND", proxy!="stats"}) by (proxy) == 0
+        max(haproxy_active_servers{sv="BACKEND", proxy!~"(stats|openstack_web_proxy-backend)"}) by (proxy)
+        + max(haproxy_backup_servers{sv="BACKEND", proxy!~"(stats|openstack_web_proxy-backend)"}) by (proxy) == 0
       labels:
         severity: critical
         service: haproxy