remove the namespace dependency of BENCHMARK_COUNTERS
remove the namespace dependency of BENCHMARK_COUNTERS

Summary: BENCHMARK_COUNTERS tests need to set `using folly::UserCounters` or `using namespace folly`. Add absolute namespace path in the macro to get rid of this requirement.

Reviewed By: Orvid

Differential Revision: D18271700

fbshipit-source-id: 084041142b7c245e3c861943570416ac89ea6c34

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly