feat: implement nixos-exporter

Prometheus exporter for NixOS-specific metrics including:
- Generation collector: count, current, booted, age, config mismatch
- Flake collector: input age, input info, revision behind

Includes NixOS module, flake packaging, and documentation.

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

module.nix

@@ -0,0 +1,122 @@
+{ self }:
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.prometheus.exporters.nixos;
+in
+{
+  options.services.prometheus.exporters.nixos = {
+    enable = lib.mkEnableOption "NixOS Prometheus exporter";
+
+    port = lib.mkOption {
+      type = lib.types.port;
+      default = 9971;
+      description = "Port to listen on.";
+    };
+
+    listenAddress = lib.mkOption {
+      type = lib.types.str;
+      default = "0.0.0.0";
+      description = "Address to listen on.";
+    };
+
+    flake = {
+      enable = lib.mkEnableOption "flake collector";
+
+      url = lib.mkOption {
+        type = lib.types.str;
+        default = "";
+        description = ''
+          Flake URL for revision comparison.
+          Required if flake collector is enabled.
+        '';
+      };
+
+      checkInterval = lib.mkOption {
+        type = lib.types.str;
+        default = "1h";
+        description = "Interval between remote flake checks.";
+      };
+    };
+
+    openFirewall = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = "Open the firewall for the exporter port.";
+    };
+
+    user = lib.mkOption {
+      type = lib.types.str;
+      default = "nixos-exporter";
+      description = "User to run the exporter as.";
+    };
+
+    group = lib.mkOption {
+      type = lib.types.str;
+      default = "nixos-exporter";
+      description = "Group to run the exporter as.";
+    };
+
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = self.packages.${pkgs.system}.default;
+      description = "The nixos-exporter package to use.";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = cfg.flake.enable -> cfg.flake.url != "";
+        message = "services.prometheus.exporters.nixos.flake.url must be set when flake collector is enabled";
+      }
+    ];
+
+    users.users.${cfg.user} = {
+      isSystemUser = true;
+      group = cfg.group;
+      description = "NixOS exporter user";
+    };
+
+    users.groups.${cfg.group} = { };
+
+    systemd.services.prometheus-nixos-exporter = {
+      description = "Prometheus NixOS Exporter";
+      wantedBy = [ "multi-user.target" ];
+      after = [ "network.target" ];
+
+      serviceConfig = {
+        User = cfg.user;
+        Group = cfg.group;
+        ExecStart = lib.concatStringsSep " " ([
+          "${cfg.package}/bin/nixos-exporter"
+          "--listen=${cfg.listenAddress}:${toString cfg.port}"
+        ] ++ lib.optionals cfg.flake.enable [
+          "--collector.flake"
+          "--flake.url=${cfg.flake.url}"
+          "--flake.check-interval=${cfg.flake.checkInterval}"
+        ]);
+        Restart = "on-failure";
+        RestartSec = "5s";
+
+        # Hardening
+        NoNewPrivileges = true;
+        ProtectSystem = "strict";
+        ProtectHome = true;
+        PrivateTmp = true;
+        PrivateDevices = true;
+        ProtectKernelTunables = true;
+        ProtectKernelModules = true;
+        ProtectControlGroups = true;
+        RestrictAddressFamilies = [ "AF_INET" "AF_INET6" ];
+        RestrictNamespaces = true;
+        RestrictRealtime = true;
+        RestrictSUIDSGID = true;
+        MemoryDenyWriteExecute = true;
+        LockPersonality = true;
+      };
+    };
+
+    networking.firewall.allowedTCPPorts = lib.mkIf cfg.openFirewall [ cfg.port ];
+  };
+}