review classifier: export request, verdict, check and run metrics, with a dashboard

The controller counts review requests, verdicts and per-check results and times each run; the classifier service counts retried asks. A provisioned Grafana dashboard charts them.

nix/host-modules/swarm-grafana.nix

@@ -278,6 +278,7 @@ let
         "logstore.json"
         "metricstore.json"
         "queue.json"
+        "review-classifier.json"
       ]
   );