defiant: add uptime-kuma, rename metrics->monitoring

hosts/defiant/services/monitoring/snmp-exporter.nix

@@ -0,0 +1,20 @@
+{ config, pkgs, ... }:
+
+{
+  environment.systemPackages = [
+    pkgs.prometheus-snmp-exporter
+  ];
+
+  systemd.services.prometheus-snmp-exporter = {
+    enable = true;
+    description = "Gather data from SNMP devices and expose them as Prometheus metrics";
+    unitConfig = {
+      Type = "simple";
+    };
+    serviceConfig = {
+      ExecStart = "${pkgs.prometheus-snmp-exporter}/bin/snmp_exporter --config.file='/tank/services/metrics/prometheus/snmp.yml'";
+      # snmp.yml = https://github.com/prometheus/snmp_exporter/blob/main/snmp.yml + https://global.download.synology.com/download/Document/Software/DeveloperGuide/Firmware/DSM/All/enu/Synology_DiskStation_MIB_Guide.pdf
+    };
+    wantedBy = [ "multi-user.target" ];
+  };
+}