Fix HaproxyBackendMaxActiveSession: look at current / limit

[fschlich]

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.

[samber]

Thanks for the fix and welcome in the contributors \o/

[DonPerinon]

The question is, what is the actual purpose of this alert? One possible use is certainly the detection of connection limits. However, in this setting, we will probably lose the ability to detect the peak number of connections if we adjust it between scraping intervals. But it likely depends on what the alert is used for.