docs: add dashboard screenshot (#161)

Co-authored-by: dengbo <[email protected]>

Author: hipibo

zh_CN/admin/system-monitor-grafana.md

@@ -75,3 +75,7 @@ docker run --name my-grafana --network host grafana/grafana
 ### 添加指标面板
 
 添加 Prometheus 数据源后，可在 `Grafana - Dashboards` 手动添加指标面板，或<a href="https://github.com/datalayers-io/datalayers-with-grafana/blob/main/grafana/dashboard-imported.json" download="datalayers_dashboard-imported.json">点击下载</a> json 文件快速导入我们提供的模版。
+
+以下为监控面板示意图：
+
+![metrics example](../assets/dashboard_metrics.jpg)