Horizon isn't really about tracking the users and their activity as I am sure you know, but depending on the license version you have there are tools like vRops that will log metrics about the sessions. You could see if certain windows processes are running within a session, i've not done that myself.
The other part is within the GPO you used to be able to activate the vCenter console into a session, we used that before you was able to share the session. But, we discovered it was not always being used for the correct reasons.
You need something like liquidware labs stratusphere or control up. You can get control up through VMware as an addon to horizon now. vRops is more about the quality of the session and not the length, as it only reports on active sessions.