[skip ci]Fix grafana expr (#10772)

Signed-off-by: NWeida Zhu <weida.zhu@zilliz.com>

deployments/monitor/grafana/milvus-dashboard.json

@@ -3126,7 +3126,7 @@
       "steppedLine": false,
       "targets": [
         {
-          "expr": "sum(rate(milvus_rootcoord_describe_segment_total{app_kubernetes_io_instance=~\"$instance\", app_kubernetes_io_name=\"$app_name\", namespace=\"$namespace\", type=\"total\"}[5m])) by (pod)",
+          "expr": "sum(increase(milvus_rootcoord_describe_segment_total{app_kubernetes_io_instance=~\"$instance\", app_kubernetes_io_name=\"$app_name\", namespace=\"$namespace\", type=\"total\"}[2m])/2)",
           "interval": "",
           "legendFormat": "total",
           "queryType": "randomWalk",