nixos/prometheus: add nvidia-gpu exporter

This exporter allows prometheus-nvidia-gpu-exporter to be installed as a
systemd service.

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -42,6 +42,8 @@
 
 - [mqtt-exporter](https://github.com/kpetremann/mqtt-exporter/), a Prometheus exporter for exposing messages from MQTT. Available as [services.prometheus.exporters.mqtt](#opt-services.prometheus.exporters.mqtt.enable).
 
+- [nvidia-gpu](https://github.com/utkuozdemir/nvidia_gpu_exporter), a Prometheus exporter that scrapes `nvidia-smi` for GPU metrics. Available as [services.prometheus.exporters.nvidia-gpu](#opt-services.prometheus.exporters.nvidia-gpu.enable).
+
 - [Buffyboard](https://gitlab.postmarketos.org/postmarketOS/buffybox/-/tree/master/buffyboard), a framebuffer on-screen keyboard. Available as [services.buffyboard](option.html#opt-services.buffyboard).
 
 - [KanBoard](https://github.com/kanboard/kanboard), a project management tool that focuses on the Kanban methodology. Available as [services.kanboard](#opt-services.kanboard.enable).

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

@@ -66,6 +66,7 @@ let
     "nginxlog"
     "node"
     "nut"
+    "nvidia-gpu"
     "pgbouncer"
     "php-fpm"
     "pihole"

nixos/modules/services/monitoring/prometheus/exporters/nvidia-gpu.nix

@@ -0,0 +1,29 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  cfg = config.services.prometheus.exporters.nvidia-gpu;
+  inherit (lib)
+    types
+    concatStringsSep
+    ;
+in
+{
+  port = 9835;
+  serviceOpts = {
+    serviceConfig = {
+      ExecStart = ''
+        ${pkgs.prometheus-nvidia-gpu-exporter}/bin/nvidia_gpu_exporter \
+          --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
+          --nvidia-smi-command ${config.hardware.nvidia.package.bin}/bin/nvidia-smi \
+          ${concatStringsSep " " cfg.extraFlags}
+      '';
+      PrivateDevices = false;
+    };
+    wantedBy = [ "multi-user.target" ];
+  };
+}