fix: ${DS_PROMETHEUS} was not found (#337)

Co-authored-by: soulbird <zhaothree@gmail.com>
diff --git a/example/grafana_conf/dashboards/apisix-grafana-dashboard.json b/example/grafana_conf/dashboards/apisix-grafana-dashboard.json
index e95974b..7f80be5 100644
--- a/example/grafana_conf/dashboards/apisix-grafana-dashboard.json
+++ b/example/grafana_conf/dashboards/apisix-grafana-dashboard.json
@@ -1630,6 +1630,16 @@
   "tags": [],
   "templating": {
     "list": [
+       {
+        "hide": 0,
+        "label": "datasource",
+        "name": "DS_PROMETHEUS",
+        "options": [],
+        "query": "prometheus",
+        "refresh": 1,
+        "regex": "apisix",
+        "type": "datasource"
+      },
       {
         "allValue": ".*",
         "current": {},