by Roadblock » Mon Jan 11, 2016 2:21 pm
I don't have access to the client at the moment but the option is under Interface Options > Advanced > something statusbar text.
It will not affect target or ungrouped players as that information wasn't available to the client in Vanilla, but it should work in groups.
Mobhealth3 (addon) can show absolute values but it needs to learn them first (it maintains and updates a database of unit/level/hp values)