This addon does not take into account availability messages from HAProxy such as these ones:
Server frontend/backend is DOWN, reason: Layer4 connection problem, info: "Connection refused", check duration: 23ms. 1 active and 4 backup servers left. 3 sessions active, 2 requeued, 24 remaining in queue.
There is no eventttype nor field extraction. However, it could be quite useful to determine availability of your server infrastructure (the DOWN/UP messages could be grouped into transactions).
Maybe an idea for future versions of this addon ?