In monolog there is already a slack integration using the api but that takes up an integration slot. If you are on the free plan you only have limited integration slots (10). In order to be able to use slack for monolog I implemented it using webhooks.
As simple as pushing your \Pageon\SlackWebhookMonolog\Monolog\SlackWebhookHandler
as a handler to Monolog
For an example of an implementation see implementation example