Improve client stats
Improve client stats

Test Plan: unit test

Reviewed By: @​zhuohuang

Differential Revision: D2203256
glensc/mcrouter,is00hcw/mcrouter,reddit/mcrouter,facebook/mcrouter,synecdoche/mcrouter,facebook/mcrouter,tempbottle/mcrouter,yqzhang/mcrouter,zhlong73/mcrouter,glensc/mcrouter,reddit/mcrouter,easyfmxu/mcrouter,nvaller/mcrouter,nvaller/mcrouter,zhlong73/mcrouter,synecdoche/mcrouter,zhlong73/mcrouter,glensc/mcrouter,nvaller/mcrouter,is00hcw/mcrouter,easyfmxu/mcrouter,tempbottle/mcrouter,is00hcw/mcrouter,yqzhang/mcrouter,facebook/mcrouter,is00hcw/mcrouter,synecdoche/mcrouter,tempbottle/mcrouter,reddit/mcrouter,nvaller/mcrouter,zhlong73/mcrouter,easyfmxu/mcrouter,yqzhang/mcrouter,yqzhang/mcrouter,facebook/mcrouter,reddit/mcrouter,easyfmxu/mcrouter,glensc/mcrouter,synecdoche/mcrouter,tempbottle/mcrouter