Fix plugin success rate logging
Fix plugin success rate logging

Summary:
I noticed that during a refactor, plugin success-rate failures started getting reported as successes.
Thankfully it's only the plugin metrics that are affected.

Reviewed By: passy

Differential Revision: D15394864

fbshipit-source-id: d9f44a0dd61f68dcdcbf2eb524821a3eb023231b

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper