prometheus-exporters: add dovecot exporter module

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

@@ -20,6 +20,7 @@ let
   exporterOpts = {
     blackbox = import ./exporters/blackbox.nix { inherit config lib pkgs; };
     collectd = import ./exporters/collectd.nix { inherit config lib pkgs; };
+    dovecot  = import ./exporters/dovecot.nix  { inherit config lib pkgs; };
     fritzbox = import ./exporters/fritzbox.nix { inherit config lib pkgs; };
     json     = import ./exporters/json.nix     { inherit config lib pkgs; };
     minio    = import ./exporters/minio.nix    { inherit config lib pkgs; };

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

@@ -0,0 +1,50 @@
+{ config, lib, pkgs }:
+
+with lib;
+
+let
+  cfg = config.services.prometheus.exporters.dovecot;
+in
+{
+  port = 9166;
+  extraOpts = {
+    telemetryPath = mkOption {
+      type = types.str;
+      default = "/metrics";
+      description = ''
+        Path under which to expose metrics.
+      '';
+    };
+    socketPath = mkOption {
+      type = types.path;
+      default = "/var/run/dovecot/stats";
+      example = "/var/run/dovecot2/stats";
+      description = ''
+        Path under which the stats socket is placed.
+        The user/group under which the exporter runs,
+        should be able to access the socket in order
+        to scrape the metrics successfully.
+      '';
+    };
+    scopes = mkOption {
+      type = types.listOf types.str;
+      default = [ "user" ];
+      example = [ "user" "global" ];
+      description = ''
+        Stats scopes to query.
+      '';
+    };
+  };
+  serviceOpts = {
+    serviceConfig = {
+      ExecStart = ''
+        ${pkgs.prometheus-dovecot-exporter}/bin/dovecot_exporter \
+          --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
+          --web.telemetry-path ${cfg.telemetryPath} \
+          --dovecot.socket-path ${cfg.socketPath} \
+          --dovecot.scopes ${concatStringsSep "," cfg.scopes} \
+          ${concatStringsSep " \\\n  " cfg.extraFlags}
+      '';
+    };
+  };
+}