Add matching engine analytics for count and average match ratings per user

[vingkan]

Goal

To get better insight on our matching engine's performance, we want to calculate, for each user, the average match rating. This will calculate the average rating a user is giving for a match. Which will allow us to see how well our matches are doing.

The formula for selection rate is:

              total match ratings for a user
    -----------------------------------
             number of match ratings for a user

##Resources
Design Doc

Definition of Done

• Implement metric function to measure the average match ratings per user
• Be able to run a script that does this