Stefano Alletti has written up a post to his site showing how to combine Symfony and Monolog along with a custom "processor" to modify the message and content being logged.
We often have to use different micro-services who write in many log files. Use utilities like Kibana is a good thing, but in order to take full advantage of its features we have to try to standardize and normalize the logs.
The company where I work having introduced Kibana recently, he asked me to implement a proper strategy to log all the micro-services.
He starts by outlining the fields that are required to be in the log output (a great place to start) before moving into the code to implement the logging. He briefly talks about Monolog itself and how to implement it in a Symfony application. The remainder of the post includes the code to implement the logger and how to add the custom processor to modify the extra data being included in the logged data. He also goes a step further and creates a custom formatter to modify the output in the message for the customer ID and product line values.