nixos/prometheus-exporters: add zfs-siebenmann exporter

Introduces the NixOS module for the Siebenmann ZFS Prometheus exporter.
Exposes depth, fullPath, and pools options matching the exporter's CLI flags.

nixos/modules/services/monitoring/prometheus/exporters.nix

@@ -131,6 +131,7 @@ let
         "v2ray"
         "varnish"
         "wireguard"
+        "zfs-siebenmann"
         "zfs"
       ]
       (

nixos/modules/services/monitoring/prometheus/exporters/zfs-siebenmann.nix

@@ -0,0 +1,64 @@
+{
+  config,
+  lib,
+  pkgs,
+  options,
+  ...
+}:
+
+let
+  cfg = config.services.prometheus.exporters.zfs-siebenmann;
+  inherit (lib)
+    mkOption
+    types
+    concatStringsSep
+    optionalString
+    ;
+in
+{
+  port = 9700;
+
+  extraOpts = {
+    pools = mkOption {
+      type = with types; listOf str;
+      default = [ ];
+      description = ''
+        Name of the pool(s) to collect, repeat for multiple pools (default: all pools).
+      '';
+    };
+
+    depth = mkOption {
+      type = types.int;
+      default = 1;
+      description = ''
+        Depth of the vdev tree to report on.
+        0 is the pool, 1 is top level vdevs, 2 is devices too.
+      '';
+    };
+
+    fullPath = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Report the full path of disks.
+      '';
+    };
+  };
+
+  serviceOpts = {
+    # needs zpool
+    path = [ config.boot.zfs.package ];
+    serviceConfig = {
+      ExecStart = ''
+        ${pkgs.prometheus-siebenmann-zfs-exporter}/bin/zfs_exporter \
+          --listen-addr ${cfg.listenAddress}:${toString cfg.port} \
+          --depth ${toString cfg.depth} \
+          ${optionalString cfg.fullPath "--fullpath"} \
+          ${concatStringsSep " " (map (p: "--pool=${p}") cfg.pools)} \
+          ${concatStringsSep " \\\n  " cfg.extraFlags}
+      '';
+      ProtectClock = false;
+      PrivateDevices = false;
+    };
+  };
+}