Simplify MemoryUsageProcessor

Analog https://github.com/Seldaek/monolog/pull/594/files

src/Monolog/Processor/MemoryUsageProcessor.php

@@ -28,12 +28,7 @@ class MemoryUsageProcessor extends MemoryProcessor
         $bytes = memory_get_usage($this->realUsage);
         $formatted = $this->formatBytes($bytes);
 
-        $record['extra'] = array_merge(
-            $record['extra'],
-            array(
-                'memory_usage' => $formatted,
-            )
-        );
+        $record['extra']['memory_usage'] = $formatted;
 
         return $record;
     }