Fix HaproxyBackendMaxActiveSession: look at current / limit (#413)

haproxy_backend_max_sessions is the maximum number of sessions ever encountered during the lifetime of the HAProxy process. That is, it will never go down until HAProxy is restarted, so the alert continues to fire even though the situation has cleared!

This doesn't make sense. Look at the currently active sessions instead.

_data/rules.yml

@@ -1785,7 +1785,7 @@ groups:
                 severity: critical
               - name: HAProxy backend max active session
                 description: HAproxy backend {{ $labels.fqdn }}/{{ $labels.backend }} is reaching session limit (> 80%).
-                query: "((sum by (backend) (avg_over_time(haproxy_backend_max_sessions[2m]) * 100) / sum by (backend) (avg_over_time(haproxy_backend_limit_sessions[2m])))) > 80"
+                query: "((sum by (backend) (avg_over_time(haproxy_backend_current_sessions[2m]) * 100) / sum by (backend) (avg_over_time(haproxy_backend_limit_sessions[2m])))) > 80"
                 severity: warning
                 for: 2m
               - name: HAProxy pending requests