protected function getCoreMetrics(ResponseAdapter $lukeData, array $fields)
    {
        $coreMetrics = [
            'numberOfDocuments' => $lukeData->index->numDocs,
            'numberOfDeletedDocuments' => $lukeData->index->deletedDocs,
            'numberOfTerms' => $lukeData->index->numTerms,
            'numberOfFields' => count($fields)
        ];

        return $coreMetrics;
    }