Allow async beforeSend function. Closes #3584

src/tracker/index.js

@@ -143,7 +143,7 @@
     const callback = window[beforeSend];
 
     if (typeof callback === 'function') {
-      payload = callback(type, payload);
+      payload = await Promise.resolve(callback(type, payload));
     }
 
     if (!payload) return;