serverAddress(app, path) {
    const addr = app.address();

    if (!addr) this._server = app.listen(0);
    const port = app.address().port;
    const protocol = app instanceof Server ? 'https' : 'http';
    return protocol + '://127.0.0.1:' + port + path;
  }
Returns a URL, extracted from a server.

@param {Server} app
@param {String} path
@returns {String} URL address
@api private