Expose ExecutorObserver outside of Mcrouter
Expose ExecutorObserver outside of Mcrouter

Summary: Allows this to be `#include`d elsewhere.

Reviewed By: ghostonhuang

Differential Revision: D32813944

fbshipit-source-id: 4c5e234cea5ba7b97f0365aeb841f1bb002fd289

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