Add some return type hints and fix up HandlerWrapper

src/Monolog/Handler/FleepHookHandler.php

@@ -11,6 +11,7 @@
 
 namespace Monolog\Handler;
 
+use Monolog\Formatter\FormatterInterface;
 use Monolog\Formatter\LineFormatter;
 use Monolog\Logger;
 
@@ -63,7 +64,7 @@ class FleepHookHandler extends SocketHandler
      *
      * @return LineFormatter
      */
-    protected function getDefaultFormatter()
+    protected function getDefaultFormatter(): FormatterInterface
     {
         return new LineFormatter(null, null, true, true);
     }