/usr/share/php/Monolog/Formatter/ is in php-monolog 1.17.2-1ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/share/php/Monolog/Formatter/ChromePHPFormatter.php
/usr/share/php/Monolog/Formatter/ElasticaFormatter.php
/usr/share/php/Monolog/Formatter/FlowdockFormatter.php
/usr/share/php/Monolog/Formatter/FormatterInterface.php
/usr/share/php/Monolog/Formatter/GelfMessageFormatter.php
/usr/share/php/Monolog/Formatter/HtmlFormatter.php
/usr/share/php/Monolog/Formatter/JsonFormatter.php
/usr/share/php/Monolog/Formatter/LineFormatter.php
/usr/share/php/Monolog/Formatter/LogglyFormatter.php
/usr/share/php/Monolog/Formatter/LogstashFormatter.php
/usr/share/php/Monolog/Formatter/MongoDBFormatter.php
/usr/share/php/Monolog/Formatter/NormalizerFormatter.php
/usr/share/php/Monolog/Formatter/ScalarFormatter.php
/usr/share/php/Monolog/Formatter/WildfireFormatter.php