Client/topology socket stats

mgo provides a mechanism to return various stats (https://godoc.org/labix.org/v2/mgo#Stats), particularly around number of in-use and idle sockets. I think you could get total sockets if you implemented a custom Dialer, but I don't think you could differentiate idle sockets.

mgo also tracks some stats on ops sent and docs received, but I think the existing CommandMonitor interface is flexible enough that'd it'd be better to just use that. ConnectionOption could be similarly extended to support a "WithServerPoolMonitor" or something like that.