Update MemoryPeakUsageProcessor.php

more simple implementation

src/Monolog/Processor/MemoryPeakUsageProcessor.php

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