nixos/alloy: init module

This adds a NixOS module for Grafana Alloy.

I started from the grafana-agent one but dropped all settings and config
management whatsoever.

Grafana Alloy uses its own Alloy config format (similar to HCL), which
is not really possible to express in Nix.
Simply pointing to a path in `/etc`, and leaving it up to the user to configure
it via `environment.etc` allows the user to arrange config files however
it makes most sense for them.

The module, systemd unit etc is called "alloy", not "grafana-alloy" to
follow the way it's packaged on other distros, to follow POLA.

nixos/modules/module-list.nix

@@ -834,6 +834,7 @@
   ./services/misc/zoneminder.nix
   ./services/misc/zookeeper.nix
   ./services/monitoring/alerta.nix
+  ./services/monitoring/alloy.nix
   ./services/monitoring/apcupsd.nix
   ./services/monitoring/arbtt.nix
   ./services/monitoring/below.nix

nixos/modules/services/monitoring/alloy.nix

@@ -0,0 +1,80 @@
+{ lib, pkgs, config, ... }:
+with lib;
+let
+  cfg = config.services.alloy;
+in
+{
+  meta = {
+    maintainers = with maintainers; [ flokli ];
+  };
+
+  options.services.alloy = {
+    enable = mkEnableOption "Grafana Alloy";
+
+    package = mkPackageOption pkgs "grafana-alloy" { };
+
+    configPath = mkOption {
+      type = lib.types.path;
+      default = "/etc/alloy";
+      description = ''
+        Alloy configuration file/directory path.
+
+        We default to `/etc/alloy` here, and expect the user to configure a
+        configuration file via `environment.etc."alloy/config.alloy"`.
+
+        This allows config reload, contrary to specifying a store path.
+        A `reloadTrigger` for `config.alloy` is configured.
+
+        Other `*.alloy` files in the same directory (ignoring subdirs) are also
+        honored, but it's necessary to manually extend
+        `systemd.services.alloy.reloadTriggers` to enable config reload
+        during nixos-rebuild switch.
+
+        This can also point to another directory containing `*.alloy` files, or
+        a single Alloy file in the Nix store (at the cost of reload).
+
+        Component names must be unique across all Alloy configuration files, and
+        configuration blocks must not be repeated.
+
+        Alloy will continue to run if subsequent reloads of the configuration
+        file fail, potentially marking components as unhealthy depending on
+        the nature of the failure. When this happens, Alloy will continue
+        functioning in the last valid state.
+      '';
+    };
+
+    extraFlags = mkOption {
+      type = with lib.types; listOf str;
+      default = [ ];
+      example = [ "--server.http.listen-addr=127.0.0.1:12346" "--disable-reporting" ];
+      description = ''
+        Extra command-line flags passed to {command}`alloy run`.
+
+        See <https://grafana.com/docs/alloy/latest/reference/cli/run/>
+      '';
+    };
+  };
+
+
+  config = mkIf cfg.enable {
+    systemd.services.alloy = {
+      wantedBy = [ "multi-user.target" ];
+      reloadTriggers = [ config.environment.etc."alloy/config.alloy".source or null ];
+      serviceConfig = {
+        Restart = "always";
+        DynamicUser = true;
+        RestartSec = 2;
+        SupplementaryGroups = [
+          # allow to read the systemd journal for loki log forwarding
+          "systemd-journal"
+        ];
+        ExecStart = "${lib.getExe cfg.package} run ${cfg.configPath} ${escapeShellArgs cfg.extraFlags}";
+        ExecReload = "${pkgs.coreutils}/bin/kill -SIGHUP $MAINPID";
+        ConfigurationDirectory = "alloy";
+        StateDirectory = "alloy";
+        WorkingDirectory = "%S/alloy";
+        Type = "simple";
+      };
+    };
+  };
+}