Monitoring HAProxy?

Yes, the only datapoint that DataSource tracks is sessions. It would seem the active discovery is not returning anything in your case. Navigate to the DataSource (the same place you tested the AppliesTo) and click the "Test Active Discovery" button. If you see no results there, that's your problem. 

This DS uses the HTTP discovery method, meaning that discovery involves pulling up a web page and scraping it for the instances (that's the word you were looking for). In this case, it's looking at https://[hostname/ip]/haproxy?stats and scrapes looking for anything matching RegEx: <th colspan=2 class=.pxname.>(.*?)</th>. I would start by hitting one of your stats enabled frontend on one of the haproxies to see if the page loads. If it does not, you probably need to add that frontend to your haproxy config.

It's possible that this used to be enabled out of the box for older versions of haproxy and the newest version of haproxy requires you to explicitly configure it.

Once you get the page loading in your browser, you might need to make some changes to the DS to get the discovery to pull the page correctly. Once that's working, it looks like it shouldn't be hard to get the other stats from the table on that page. You'll just have to get real familiar with RegEx.

I just got haproxy up and running in Docker and i'll take a look today during any free time i have to see what can be done to pull some of the other stats. Did you manually add the haproxy category to your servers or was it discovered? I'm not aware of a propertysource that auto-discovers haproxy installed on devices, but it wouldn't be the first time there's a propertysource i'm unaware of.