yanzong

Monolog\Logger::API can be used to distinguish between a Monolog 1 and 2 install of Monolog when writing integration code.
Removed non-PSR-3 methods to add records, all the add* (e.g. addWarning) methods as well as emerg, crit, err and warn.
DateTime are now formatted with a timezone and microseconds (unless disabled). Various formatters and log output might be affected, which may mess with log parsing in some cases.
The datetime in every record array is now a DateTimeImmutable, not that you should have been modifying these anyway.
The timezone is now set per Logger instance and not statically, either via ->setTimezone or passed in the constructor. Calls to Logger::setTimezone should be converted.
HandlerInterface has been split off and two new interfaces now exist for more granular controls: ProcessableHandlerInterface and FormattableHandlerInterface. Handlers not extending AbstractHandler should make sure to implement the relevant interfaces.
HandlerInterface now requires the close method to be implemented. This only impacts you if you implement the interface yourself, but you can extend the new Monolog\Handler\Handler base class too.
There is no more default handler configured on empty Logger instances, if you were relying on that you will not get any output anymore, make sure to configure the handler you need.

The records' datetime is not sent anymore. Only timestamp is sent to Loggly.

Log levels are not shortened to 4 characters anymore. e.g. a warning record will be sent using the warning.channel routing key instead of warn.channel as in 1.x.
The exchange name does not default to 'log' anymore, and it is completely ignored now for the AMQP extension users. Only PHPAmqpLib uses it if provided.

The file name format must now contain {date} and the date format must be set to one of the predefined FILE_PER_* constants to avoid issues with file rotation. See setFilenameFormat.

Removed Logstash V0 support
Context/extra prefix has been removed in favor of letting users configure the exact key being sent
Context/extra data are now sent as an object instead of single keys

Removed deprecated HipChat handler, migrate to Slack and use SlackWebhookHandler or SlackHandler instead

Removed deprecated SlackbotHandler handler, use SlackWebhookHandler or SlackHandler instead

Removed deprecated RavenHandler handler, use sentry/sentry 2.x and their Sentry\Monolog\Handler instead

As support for the official Elasticsearch library was added, the former ElasticSearchHandler has been renamed to ElasticaHandler and the new one added as ElasticsearchHandler.