fix(monitoring): order nvidia exporter after network-online

The nvidia exporter binds the host's static LAN IPv4, but unlike the node and smartctl exporters it was not ordered after network-online.target. On boot it lost the race with networkd, hit the systemd start limit, and stayed failed until manually restarted.

modules/nixos/base/monitoring.nix

@@ -87,6 +87,7 @@ in
     lib.genAttrs
       (
         [ "prometheus-node-exporter" ]
+        ++ lib.optional config.services.prometheus.exporters.nvidia-gpu.enable "prometheus-nvidia-gpu-exporter"
         ++ lib.optionals config.modules.hardwareTools.enable [ "prometheus-smartctl-exporter" ]
       )
       (_: {