For HA proxy you can use either socket connection (default) or URL connection for the stats. Since you mention an appliance - you will probably need to configure the HA proxy to have stats exposed.
For details see documentation or agent plugin code agent/plugin/plugins/ruxit.python.haproxy/ruxit_haproxy.py
As @James K. mentions below, plugins can currently not communicate outside of the host they are running on and thus require a oneagent.
Lateron, remote plugins could support this usecase as that requirement for running on the same machine is not there.