Logger: rename level => levelname to avoid name collision

lib/utils/logger.js

@@ -18,10 +18,10 @@ module.exports = class Logger {
             formatters: {
                 level (label) {
                     if (label === 'warn') {
-                        return { level: 'warning' };
+                        return { levelname: 'warning' };
                     }
 
-                    return { level: label };
+                    return { levelname: label };
                 }
             },
             messageKey: 'message',