protected function prepareCoreMetricsArchive($visits, $visitsConverted)
    {
        $createSeparateArchiveForCoreMetrics = $this->mustProcessVisitCount($visits)
                                && !$this->doesRequestedPluginIncludeVisitsSummary();

        if ($createSeparateArchiveForCoreMetrics) {
            $requestedPlugin = $this->params->getRequestedPlugin();

            $this->params->setRequestedPlugin('VisitsSummary');

            $pluginsArchiver = new PluginsArchiver($this->params, $this->isArchiveTemporary());
            $metrics = $pluginsArchiver->callAggregateCoreMetrics();
            $pluginsArchiver->finalizeArchive();

            $this->params->setRequestedPlugin($requestedPlugin);

            $visits = $metrics['nb_visits'];
            $visitsConverted = $metrics['nb_visits_converted'];
        }

        return array($visits, $visitsConverted);
    }