Fix system dashboard $node template variable queries

The label_values() PromQL function requires a metric name as the first
argument. Without it, Prometheus returns raw label hashes instead of
readable node names like "validator-0:6006".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -501,7 +501,7 @@
         "label": "Node",
         "description": "Filter by rippled node (service.instance.id)",
         "type": "query",
-        "query": "label_values(exported_instance)",
+        "query": "label_values(rippled_ledger_data_get_Bytes_In, exported_instance)",
         "datasource": {
           "type": "prometheus",
           "uid": "prometheus"

docker/telemetry/grafana/dashboards/system-network-traffic.json

@@ -666,7 +666,7 @@
         "label": "Node",
         "description": "Filter by rippled node (service.instance.id)",
         "type": "query",
-        "query": "label_values(exported_instance)",
+        "query": "label_values(rippled_Peer_Finder_Active_Inbound_Peers, exported_instance)",
         "datasource": {
           "type": "prometheus",
           "uid": "prometheus"

docker/telemetry/grafana/dashboards/system-node-health.json

@@ -410,7 +410,7 @@
         "label": "Node",
         "description": "Filter by rippled node (service.instance.id)",
         "type": "query",
-        "query": "label_values(exported_instance)",
+        "query": "label_values(rippled_LedgerMaster_Validated_Ledger_Age, exported_instance)",
         "datasource": {
           "type": "prometheus",
           "uid": "prometheus"

docker/telemetry/grafana/dashboards/system-overlay-traffic-detail.json

@@ -561,7 +561,7 @@
         "label": "Node",
         "description": "Filter by rippled node (service.instance.id)",
         "type": "query",
-        "query": "label_values(exported_instance)",
+        "query": "label_values(rippled_squelch_Messages_In, exported_instance)",
         "datasource": {
           "type": "prometheus",
           "uid": "prometheus"

docker/telemetry/grafana/dashboards/system-rpc-pathfinding.json

@@ -391,7 +391,7 @@
         "label": "Node",
         "description": "Filter by rippled node (service.instance.id)",
         "type": "query",
-        "query": "label_values(exported_instance)",
+        "query": "label_values(rippled_rpc_requests_total, exported_instance)",
         "datasource": {
           "type": "prometheus",
           "uid": "prometheus"