update pushgateway docs

roles/pushgateway/meta/argument_specs.yml

@@ -32,6 +32,16 @@ argument_specs:
       pushgateway_web_telemetry_path:
         description: "Path under which to expose metrics"
         default: "/metrics"
+      pushgateway_web_external_url:
+        description: >
+          External address on which pushgateway is available.
+          Useful when behind reverse proxy, e.g.: https://example.org/pushgateway
+        default: ""
+      pushgateway_cli_extra_flags:
+        description: >
+          Optional extra flags to be passed to pushgateway binary, e.g.
+          ['--log.level=debug', '--log.format=json', '--web.enable-admin-api']
+        default: []
       pushgateway_tls_server_config:
         description:
           - "Configuration for TLS authentication."