fix(monitoring): add missing ClickHouse Prometheus port and fix dashboard queries

- Add port 9126 mapping for ClickHouse Prometheus metrics endpoint
  (was configured in prometheus_ch.xml but not exposed in docker-compose.yml)

- Fix CPU Usage panel: use delta() instead of rate() for gauge metric
  ClickHouseProfileEvents_OSCPUVirtualTimeMicroseconds is a gauge, not counter

- Add explicit datasource blocks to dashboard queries for consistency

ClickHouse ProfileEvents metrics correctly use rate() — they are counters.
Warning about missing _total suffix is expected (ClickHouse naming convention).

configs/grafana/provisioning/dashboards/clickhouse-overview.json

@@ -385,6 +385,10 @@
       },
       "targets": [
         {
+          "datasource": {
+            "type": "prometheus",
+            "uid": "prometheus_uid"
+          },
           "expr": "rate(ClickHouseProfileEvents_Query[1m])",
           "refId": "A"
         }
@@ -671,6 +675,10 @@
       },
       "targets": [
         {
+          "datasource": {
+            "type": "prometheus",
+            "uid": "prometheus_uid"
+          },
           "expr": "rate(ClickHouseProfileEvents_InsertedRows[1m])",
           "refId": "A"
         }
@@ -967,6 +975,10 @@
       },
       "targets": [
         {
+          "datasource": {
+            "type": "prometheus",
+            "uid": "prometheus_uid"
+          },
           "expr": "rate(ClickHouseProfileEvents_Merge[1m])",
           "refId": "A"
         }