From https://logstash-beta.wmflabs.org/#/dashboard/elasticsearch/fatalmonitor since Feb 12 at 20:30 UTC:
Class undefined: GWToolset\Config in /srv/mediawiki/wmf-config/CommonSettings.php on line 1902
Fatal error: Class undefined: GWToolset\Config in /srv/mediawiki/wmf-config/CommonSettings.php on line 1902
Most probably caused by https://gerrit.wikimedia.org/r/#/c/268897/