private void setActiveApp(ApplicationVersions applicationVersions) {
        ApplicationId applicationId = applicationVersions.getId();
        Collection<String> hostsForApp = applicationVersions.allHosts();
        hostRegistry.update(applicationId, hostsForApp);
        applicationVersions.updateHostMetrics();
        tenantMetricUpdater.setApplications(applicationMapper.numApplications());
        applicationMapper.register(applicationId, applicationVersions);
    }
Activates the config of the given app. Notifies listeners

@param applicationVersions the {@link ApplicationVersions} to be activated
configserver/src/main/java/com/yahoo/vespa/config/server/application/TenantApplications.java