# Note that this attribute is optional.
      extraOpts = {
        telemetryPath = lib.mkOption {
          type = lib.types.str;
          default = "/metrics";
          description = ''
            Path under which to expose metrics.
          '';
        };
        logfilePath = lib.mkOption {
          type = lib.types.path;
          default = /var/log/postfix_exporter_input.log;
          example = /var/log/mail.log;
          description = ''
            Path where Postfix writes log entries.
            This file will be truncated by this exporter!
          '';
        };
        showqPath = lib.mkOption {
          type = lib.types.path;
          default = /var/spool/postfix/public/showq;
          example = /var/lib/postfix/queue/public/showq;
          description = ''
            Path at which Postfix places its showq socket.
          '';
        };
      };

      # `serviceOpts` is an attribute set which contains configuration
      # for the exporter's systemd service. One of
      # `serviceOpts.script` and `serviceOpts.serviceConfig.ExecStart`
      # has to be specified here. This will be merged with the default
      # service configuration.
      # Note that by default 'DynamicUser' is 'true'.
      serviceOpts = {
        serviceConfig = {
          DynamicUser = false;
          ExecStart = ''
            ${pkgs.prometheus-postfix-exporter}/bin/postfix_exporter \
              --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
              --web.telemetry-path ${cfg.telemetryPath} \
              ${lib.concatStringsSep " \\\n  " cfg.extraFlags}
          '';
        };
      };
    }
    ```
  - This should already be enough for the postfix exporter. Additionally one
    could now add assertions and conditional default values. This can be done
    in the 'meta-module' that combines all exporter definitions and generates
    the submodules:
    `nixpkgs/nixos/modules/services/prometheus/exporters.nix`

## Updating an exporter module {#module-services-prometheus-exporters-update-exporter-module}

Should an exporter option change at some point, it is possible to add
information about the change to the exporter definition similar to
`nixpkgs/nixos/modules/rename.nix`:
```nix
{
  config,
  lib,
  pkgs,
  options,
}:

let
  cfg = config.services.prometheus.exporters.nginx;
in
{
  port = 9113;
  extraOpts = {
    # additional module options
    # ...
  };
  serviceOpts = {
    # service configuration
    # ...
  };
  imports = [
    # 'services.prometheus.exporters.nginx.telemetryEndpoint' -> 'services.prometheus.exporters.nginx.telemetryPath'
    (lib.mkRenamedOptionModule [ "telemetryEndpoint" ] [ "telemetryPath" ])

    # removed option 'services.prometheus.exporters.nginx.insecure'
    (lib.mkRemovedOptionModule [ "insecure" ] ''
      This option was replaced by 'prometheus.exporters.nginx.sslVerify' which defaults to true.
    '')
    ({ options.warnings = options.warnings; })
  ];
}
```