Implement Stats Service in Open Match to provide Player Pool Statistics

A Director calling into Open Match may need statistics about the current player pool size for a specific profile etc., to determine what rate to make match generation calls at.

In general, no component in the online game services system should directly connect to redis. Any information that any component may fetch from redis, we need to implemement as a first class Stats service.

Please use this issue to add any information that this stats service may need to provide.