monitoring: add nixos-exporter to monitoring01

Enable the nixos-exporter prometheus exporter with flake collector
to track NixOS generation metrics and flake revision status.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hosts/monitoring01/configuration.nix

@@ -99,6 +99,23 @@
     ];
   };
 
+  # NixOS metrics exporter
+  services.prometheus.exporters.nixos = {
+    enable = true;
+    flake = {
+      enable = true;
+      url = "git+https://git.t-juice.club/torjus/nixos-servers.git";
+    };
+  };
+
+  # Register nixos-exporter as a Prometheus scrape target
+  homelab.monitoring.scrapeTargets = [
+    {
+      job_name = "nixos-exporter";
+      port = 9971;
+    }
+  ];
+
   labmon = {
     enable = true;