fix(telemetry): declare the DS_PROMETHEUS variable ledger-data-sync references

The dashboard's template variables and panel targets select their
datasource via ${DS_PROMETHEUS}, but the variable itself was absent from
templating.list. An unresolvable datasource variable leaves those queries
without a datasource, so the panels render empty.

Add the datasource variable as the first templating entry, matching the
other dashboards in this folder.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docker/telemetry/grafana/dashboards/ledger-data-sync.json

@@ -1492,6 +1492,19 @@
   "tags": ["ledger", "sync"],
   "templating": {
     "list": [
+      {
+        "name": "DS_PROMETHEUS",
+        "type": "datasource",
+        "label": "Prometheus",
+        "query": "prometheus",
+        "regex": "",
+        "current": {},
+        "hide": 0,
+        "refresh": 1,
+        "includeAll": false,
+        "multi": false,
+        "options": []
+      },
       {
         "name": "service_name",
         "label": "Service Name",