Add nixos module

flake.nix

@@ -35,5 +35,46 @@
       devShells.default =
         pkgs.devshell.mkShell {packages = with pkgs; [go gopls];};
       formatter = pkgs.alejandra;
-    });
+    })
+    // {
+      overlays.default = final: prev: {
+        twitter-prometheus = self.defaultPackage.${prev.system};
+      };
+      nixosModules.default = {
+        config,
+        lib,
+        pkgs,
+        ...
+      }:
+        with lib; let
+          cfg = config.services.twitter-prometheus;
+        in {
+          options.services.twitter-prometheus = {
+            enable = mkEnableOption "Enables the twitter-prometheus service";
+            listenAddr = mkOption {
+              type = types.string;
+              description = "The address to listen on.";
+              default = "localhost";
+            };
+            listenPort = mkOption {
+              type = types.port;
+              description = "The port to listen on.";
+              default = 9700;
+            };
+          };
+          config = mkIf cfg.enable {
+            nixpkgs.overlays = [self.overlays.default];
+            systemd.services.twitter-prometheus = {
+              description = self.description;
+              wantedBy = ["multi-user.target"];
+              serviceConfig = {
+                User = "twitter-prometheus";
+                ExecStart = "${pkgs.twitter-prometheus}/bin/twitter-prometheus -listenAddr ${cfg.listenAddr}:${toString cfg.listenPort}";
+                Restart = "on-failure";
+                DynamicUser = true;
+              };
+            };
+          };
+        };
+    };
 }