As per https://logstash.wikimedia.org/app/kibana#/dashboard/Fatal-Monitor?_g=h@5dd00c6&_a=h@e86ca53 we've the following errors starting and increasing from 15th December 2019:
message PHP Fatal error: Allowed memory size of 524288000 bytes exhausted (tried to allocate 20480 bytes) in /var/www/php-monitoring/lib.php on line 35