added more info to course metrics

@martynjames  @mattdrayer  added more info to course metrics like `users_started`, `grade_cutoffs`, `num_threads` and `num_active_threads`. 
Because the slash format is deprecated and will be transitioning out at some point, I think it might be better to combine this method with get_course_key rather than adding a specific method -- we could simply include "format=deprecated" as an argument and perform the necessary transformation when requested.