public function listClusters($projectId, $zone, array $optionalArgs = [])
    {
        $request = new ListClustersRequest();
        $request->setProjectId($projectId);
        $request->setZone($zone);
        if (isset($optionalArgs['parent'])) {
            $request->setParent($optionalArgs['parent']);
        }

        $requestParams = new RequestParamsHeaderDescriptor([
          'parent' => $request->getParent(),
        ]);
        $optionalArgs['headers'] = isset($optionalArgs['headers'])
            ? array_merge($requestParams->getHeader(), $optionalArgs['headers'])
            : $requestParams->getHeader();

        return $this->startCall(
            'ListClusters',
            ListClustersResponse::class,
            $optionalArgs,
            $request
        )->wait();
    }