add reporting of TTFD

Author: buenaflor

flutter/example/lib/main.dart

@@ -115,6 +115,9 @@ class MyApp extends StatefulWidget {
 class _MyAppState extends State<MyApp> {
   @override
   Widget build(BuildContext context) {
+    Future.delayed(const Duration(seconds: 3), () {
+      SentryFlutter.reportFullyDisplayed();
+    });
     return feedback.BetterFeedback(
       child: ChangeNotifierProvider<ThemeProvider>(
         create: (_) => ThemeProvider(),