Add fluentd package and module

nixos/modules/services/logging/fluentd.nix

@@ -0,0 +1,39 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.fluentd;
+in {
+  ###### interface
+
+  options = {
+
+    services.fluentd = {
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = "Whether to enable fluentd.";
+      };
+
+      config = mkOption {
+        type = types.lines;
+        default = "";
+        description = "Fluentd config.";
+      };
+    };
+  };
+
+
+  ###### implementation
+
+  config = mkIf cfg.enable {
+    systemd.services.fluentd = with pkgs; {
+      description = "Fluentd Daemon";
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        ExecStart = "${pkgs.fluentd}/bin/fluentd -c ${pkgs.writeText "fluentd.conf" cfg.config}";
+      };
+    };
+  };
+}