mtr-exporter: support specifying multiple jobs

This ability has been added in `0.3.0` release:
https://github.com/mgumz/mtr-exporter/releases/tag/0.3.0
https://github.com/NixOS/nixpkgs/pull/252667

To achieve this a config is generated and symlinked at `/etc/mtr-exporter.conf`.

Signed-off-by: Jakub Sokołowski <[e-mail]>

nixos/doc/manual/release-notes/rl-2311.section.md

···
 
 - `rome` was removed because it is no longer maintained and is succeeded by `biome`.
 
+- The `services.mtr-exporter.target` has been removed in favor of `services.mtr-exporter.jobs` which allows specifying multiple targets.
+
 ## Other Notable Changes {#sec-release-23.11-notable-changes}
 
 - The Cinnamon module now enables XDG desktop integration by default. If you are experiencing collisions related to xdg-desktop-portal-gtk you can safely remove `xdg.portal.extraPortals = [ pkgs.xdg-desktop-portal-gtk ];` from your NixOS configuration.

nixos/modules/services/networking/mtr-exporter.nix

···
 
 let
   inherit (lib)
-    maintainers types mkEnableOption mkOption mkIf
-    literalExpression escapeShellArg escapeShellArgs;
+    maintainers types literalExpression
+    escapeShellArg escapeShellArgs
+    mkEnableOption mkOption mkRemovedOptionModule mkIf mdDoc
+    optionalString concatMapStrings concatStringsSep;
+
   cfg = config.services.mtr-exporter;
+
+  jobsConfig = pkgs.writeText "mtr-exporter.conf" (concatMapStrings (job: ''
+    ${job.name} -- ${job.schedule} -- ${concatStringsSep " " job.flags} ${job.address}
+  '') cfg.jobs);
 in {
+  imports = [
+    (mkRemovedOptionModule [ "services" "mtr-exporter" "target" ] "Use services.mtr-exporter.jobs instead.")
+    (mkRemovedOptionModule [ "services" "mtr-exporter" "mtrFlags" ] "Use services.mtr-exporter.jobs.<job>.flags instead.")
+  ];
+
   options = {
     services = {
       mtr-exporter = {
-        enable = mkEnableOption (lib.mdDoc "a Prometheus exporter for MTR");
+        enable = mkEnableOption (mdDoc "a Prometheus exporter for MTR");
 
-        target = mkOption {
+        address = mkOption {
           type = types.str;
-          example = "example.org";
-          description = lib.mdDoc "Target to check using MTR.";
-        };
-
-        interval = mkOption {
-          type = types.int;
-          default = 60;
-          description = lib.mdDoc "Interval between MTR checks in seconds.";
+          default = "127.0.0.1";
+          description = lib.mdDoc "Listen address for MTR exporter.";
         };
 
         port = mkOption {
           type = types.port;
           default = 8080;
-          description = lib.mdDoc "Listen port for MTR exporter.";
+          description = mdDoc "Listen port for MTR exporter.";
         };
 
-        address = mkOption {
-          type = types.str;
-          default = "127.0.0.1";
-          description = lib.mdDoc "Listen address for MTR exporter.";
+        extraFlags = mkOption {
+          type = types.listOf types.str;
+          default = [];
+          example = ["-flag.deprecatedMetrics"];
+          description = mdDoc ''
+            Extra command line options to pass to MTR exporter.
+          '';
+        };
+
+        package = mkOption {
+          type = types.package;
+          default = pkgs.mtr-exporter;
+          defaultText = literalExpression "pkgs.mtr-exporter";
+          description = mdDoc "The MTR exporter package to use.";
+        };
+
+        mtrPackage = mkOption {
+          type = types.package;
+          default = pkgs.mtr;
+          defaultText = literalExpression "pkgs.mtr";
+          description = mdDoc "The MTR package to use.";
         };
 
-        mtrFlags = mkOption {
-          type = with types; listOf str;
-          default = [];
-          example = ["-G1"];
-          description = lib.mdDoc "Additional flags to pass to MTR.";
+        jobs = mkOption {
+          description = mdDoc "List of MTR jobs. Will be added to /etc/mtr-exporter.conf";
+          type = types.nonEmptyListOf (types.submodule {
+            options = {
+              name = mkOption {
+                type = types.str;
+                description = mdDoc "Name of ICMP pinging job.";
+              };
+
+              address = mkOption {
+                type = types.str;
+                example = "host.example.org:1234";
+                description = mdDoc "Target address for MTR client.";
+              };
+
+              schedule = mkOption {
+                type = types.str;
+                default = "@every 60s";
+                example = "@hourly";
+                description = mdDoc "Schedule of MTR checks. Also accepts Cron format.";
+              };
+
+              flags = mkOption {
+                type = with types; listOf str;
+                default = [];
+                example = ["-G1"];
+                description = mdDoc "Additional flags to pass to MTR.";
+              };
+            };
+          });
         };
       };
     };
   };
 
   config = mkIf cfg.enable {
+    environment.etc."mtr-exporter.conf" = {
+      source = jobsConfig;
+    };
+
     systemd.services.mtr-exporter = {
-      script = ''
-        exec ${pkgs.mtr-exporter}/bin/mtr-exporter \
-          -mtr ${pkgs.mtr}/bin/mtr \
-          -schedule '@every ${toString cfg.interval}s' \
-          -bind ${escapeShellArg cfg.address}:${toString cfg.port} \
-          -- \
-          ${escapeShellArgs (cfg.mtrFlags ++ [ cfg.target ])}
-      '';
       wantedBy = [ "multi-user.target" ];
       requires = [ "network.target" ];
       after = [ "network.target" ];
       serviceConfig = {
+        ExecStart = ''
+          ${cfg.package}/bin/mtr-exporter \
+            -mtr '${cfg.mtrPackage}/bin/mtr' \
+            -bind ${escapeShellArg "${cfg.address}:${toString cfg.port}"} \
+            -jobs '${jobsConfig}' \
+            ${escapeShellArgs cfg.extraFlags}
+        '';
         Restart = "on-failure";
         # Hardening
         CapabilityBoundingSet = [ "" ];