Add metrics for certain data structure sizes

There are some dictionaries that hold strong references to their values which we should post some metrics about to make sure they aren't growing without bound. Mostly I want to know "how many elements are in the dictionary".

https://github.com/FAForever/server/blob/ccf349e3b4bbaee95373d79a0e2f4ee9c5d064a2/server/ladder_service.py#L53

https://github.com/FAForever/server/blob/ccf349e3b4bbaee95373d79a0e2f4ee9c5d064a2/server/player_service.py#L25

https://github.com/FAForever/server/blob/ccf349e3b4bbaee95373d79a0e2f4ee9c5d064a2/server/game_service.py#L46

https://github.com/FAForever/server/blob/ccf349e3b4bbaee95373d79a0e2f4ee9c5d064a2/server/matchmaker/matchmaker_queue.py#L49