Add some return type hints and fix up HandlerWrapper

src/Monolog/Handler/CouchDBHandler.php

@@ -11,6 +11,7 @@
 
 namespace Monolog\Handler;
 
+use Monolog\Formatter\FormatterInterface;
 use Monolog\Formatter\JsonFormatter;
 use Monolog\Logger;
 
@@ -65,7 +66,7 @@ class CouchDBHandler extends AbstractProcessingHandler
     /**
      * {@inheritDoc}
      */
-    protected function getDefaultFormatter()
+    protected function getDefaultFormatter(): FormatterInterface
     {
         return new JsonFormatter(JsonFormatter::BATCH_MODE_JSON, false);
     }