activate NixOS monitoring in prometheus

hosts/mail-1/configuration.nix

@@ -12,5 +12,6 @@
     enable = true;
     id = "208";
     pubkey = "s2GT9iYQmuiwkZ04KzSqqlIEbUJ6CT6i4htCjWYt2xs=";
+    nixos = false;
   };
 }

hosts/monitoring-3/configuration.nix

@@ -23,6 +23,13 @@ let
     "${host.config.networking.hostName}.mon.clerie.de:9100")
     monitoringHosts;
 
+  nixosMonitoringTargets = mapAttrsToList (name: host:
+    "${host.config.networking.hostName}.mon.clerie.de:9152")
+    (filterAttrs (name: host:
+    # assume this is a NixOS system if not specified
+    attrByPath ["clerie" "monitoring" "nixos"] true host.config)
+    monitoringHosts);
+
   birdMonitoringTargets = mapAttrsToList (name: host:
     "${host.config.networking.hostName}.mon.clerie.de:9324")
     (filterAttrs (name: host:
@@ -149,6 +156,15 @@ in {
           }
         ];
       }
+      {
+        job_name = "nixos-exporter";
+        scrape_interval = "1m";
+        static_configs = [
+          {
+            targets = nixosMonitoringTargets;
+          }
+        ];
+      }
       {
         job_name = "bird-exporter";
         scrape_interval = "20s";