Add GKs to logs and error reports

Summary: To allow comparisons between metrics/errors in and out of GKs.

Reviewed By: passy

Differential Revision: D13714754

fbshipit-source-id: 311a1cd7bd5a7b2332e20a32d37d0fb323d00569

src/fb-stubs/GK.js

@@ -19,4 +19,8 @@ export default class GK {
     }
     return false;
   }
+
+  static serializeGKs() {
+    return JSON.stringify({});
+  }
 }