metrics: set objectMode to avoid lots of JSON

src/routes/system.js

@@ -149,8 +149,7 @@ async function getMetricStream(req, res, next) {
     });
     res.write('retry: 3000\n');
     res.on('close', () => metricStream.destroy());
-    metricStream.on('data', function (data) {
-        const obj = JSON.parse(data);
+    metricStream.on('data', function (obj) { // objectMode stream
         const sse = `data: ${JSON.stringify(obj)}\n\n`;
         res.write(sse);
     });