修复服务器详情不显示的问题

server/prometheus-service.js

@@ -220,6 +220,7 @@ async function getOverviewMetrics(url, sourceName) {
   const instances = new Map();
 
   const getOrCreate = (metric) => {
+    const originalInstance = metric.instance || 'Unknown';
     const job = metric.job || 'Unknown';
     const token = getServerToken(originalInstance, job, sourceName);