Fix alert on HTTP 5xx responses

Change-Id: I69ef95383a35e62715a45bcdf47714531223c471
diff --git a/haproxy/meta/prometheus.yml b/haproxy/meta/prometheus.yml
index abd9c92..4daf296 100644
--- a/haproxy/meta/prometheus.yml
+++ b/haproxy/meta/prometheus.yml
@@ -4,7 +4,7 @@
   alert:
     HAproxyHTTPResponse5xx:
       if: >-
-        irate(haproxy_http_response_5xx[1m]) > 1
+        rate(haproxy_http_response_5xx{sv="FRONTEND"}[1m]) > 1
       for: 2m
       labels:
         severity: warning