folly::format invocation issues in mcrouter project
folly::format invocation issues in mcrouter project

Summary:
In `mcrouter/stats.cpp` we have a few useless calls to folly::format (i.e. we
call it without any actual argument to format the string.

Reviewed By: andreazevedo

Differential Revision: D5913416

fbshipit-source-id: 239196cff35b3d7d02db8ba583328ef60950093d

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter