position of user in completions and course avg

@mattdrayer @martynjames  added new api to get position of user in a course in terms of course module completions and course average completions. It'll return a JSON like this
`
{
            "position": 4,
            "completions": 5,
            "course_avg": 4.5
}
`
`position` is user's position among other users
`completions` is # of course modules completed by the user
`course_avg` is avg of course module completions in the course
Might it be possible to combine this into /metrics/completions/leaders -- it seems we could return these aggregate values along with the Top N leaders, and if an additional user_id parameter is provided, we could return that user's 'user_position' value as well.