nixos: add prometheus_nginxlog_exporter module + test

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

@@ -37,6 +37,7 @@ let
     "modemmanager"
     "nextcloud"
     "nginx"
+    "nginxlog"
     "node"
     "openvpn"
     "postfix"

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

@@ -0,0 +1,51 @@
+{ config, lib, pkgs, options }:
+
+with lib;
+
+let
+  cfg = config.services.prometheus.exporters.nginxlog;
+in {
+  port = 9117;
+  extraOpts = {
+    settings = mkOption {
+      type = types.attrs;
+      default = {};
+      description = ''
+        All settings of nginxlog expressed as an Nix attrset.
+
+        Check the official documentation for the corresponding YAML
+        settings that can all be used here: https://github.com/martin-helmich/prometheus-nginxlog-exporter
+
+        The `listen` object is already generated by `port`, `listenAddress` and `metricsEndpoint` and
+        will be merged with the value of `settings` before writting it as JSON.
+      '';
+    };
+
+    metricsEndpoint = mkOption {
+      type = types.str;
+      default = "/metrics";
+      description = ''
+        Path under which to expose metrics.
+      '';
+    };
+  };
+
+  serviceOpts = let
+    listenConfig = {
+      listen = {
+        port = cfg.port;
+        address = cfg.listenAddress;
+        metrics_endpoint = cfg.metricsEndpoint;
+      };
+    };
+    completeConfig = pkgs.writeText "nginxlog-exporter.yaml" (builtins.toJSON (lib.recursiveUpdate listenConfig cfg.settings));
+  in {
+    serviceConfig = {
+      ExecStart = ''
+        ${pkgs.prometheus-nginxlog-exporter}/bin/prometheus-nginxlog-exporter -config-file ${completeConfig}
+      '';
+      Restart="always";
+      ProtectSystem="full";
+    };
+  };
+}