prometheus-postgres-exporter: add environment file (#116373)

* prometheus-postgres-exporter: add environment file

* Update nixos/modules/services/monitoring/prometheus/exporters/postgres.nix

Co-authored-by: WilliButz <WilliButz@users.noreply.github.com>

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

@@ -30,12 +30,49 @@ in
         Whether to run the exporter as the local 'postgres' super user.
       '';
     };
+
+    # TODO perhaps LoadCredential would be more appropriate
+    environmentFile = mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      example = "/root/prometheus-postgres-exporter.env";
+      description = ''
+        Environment file as defined in <citerefentry>
+        <refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum>
+        </citerefentry>.
+
+        Secrets may be passed to the service without adding them to the
+        world-readable Nix store, by specifying placeholder variables as
+        the option value in Nix and setting these variables accordingly in the
+        environment file.
+
+        Environment variables from this file will be interpolated into the
+        config file using envsubst with this syntax:
+        <literal>$ENVIRONMENT ''${VARIABLE}</literal>
+
+        The main use is to set the DATA_SOURCE_NAME that contains the
+        postgres password
+
+        note that contents from this file will override dataSourceName
+        if you have set it from nix.
+
+        <programlisting>
+          # Content of the environment file
+          DATA_SOURCE_NAME=postgresql://username:password@localhost:5432/postgres?sslmode=disable
+        </programlisting>
+
+        Note that this file needs to be available on the host on which
+        this exporter is running.
+      '';
+    };
+
   };
   serviceOpts = {
     environment.DATA_SOURCE_NAME = cfg.dataSourceName;
     serviceConfig = {
       DynamicUser = false;
       User = mkIf cfg.runAsLocalSuperUser (mkForce "postgres");
+      EnvironmentFile = mkIf (cfg.environmentFile != null) [ cfg.environmentFile ];
       ExecStart = ''
         ${pkgs.prometheus-postgres-exporter}/bin/postgres_exporter \
           --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \