[skip ci] Refine Describe Index Rate panel expression (#10761)

Signed-off-by: NEdward Zeng <jie.zeng@zilliz.com>

deployments/monitor/grafana/milvus-dashboard.json

@@ -2598,14 +2598,14 @@
       "steppedLine": false,
       "targets": [
         {
-          "expr": "sum(rate(milvus_rootcoord_describe_index_total{app_kubernetes_io_instance=~\"$instance\", app_kubernetes_io_name=\"$app_name\", namespace=\"$namespace\", type=\"total\"}[5m])) by (pod)",
+          "expr": "sum(increase(milvus_rootcoord_describe_index_total{app_kubernetes_io_instance=~\"$instance\", app_kubernetes_io_name=\"$app_name\", namespace=\"$namespace\", type=\"total\"}[2m])/2)",
           "interval": "",
           "legendFormat": "total",
           "queryType": "randomWalk",
           "refId": "A"
         },
         {
-          "expr": "sum(rate(milvus_rootcoord_describe_index_total{app_kubernetes_io_instance=~\"$instance\", app_kubernetes_io_name=\"$app_name\", namespace=\"$namespace\", type=\"success\"}[5m])) by (pod)",
+          "expr": "sum(increase(milvus_rootcoord_describe_index_total{app_kubernetes_io_instance=~\"$instance\", app_kubernetes_io_name=\"$app_name\", namespace=\"$namespace\", type=\"success\"}[2m])/2)",
           "interval": "",
           "legendFormat": "success",
           "refId": "B"