Usage of setExtraDataCallback is not very clear

[dash1291]

I'd like to use setExtraDataCallback to send extra fields in the logs, but it's not clear how to override the message object.

[dash1291]

We should at least have the callback function signature in the README.

[priyansh1161]

It is a dynamic way to set extra data with each log line. Meaning this function will be called each time a new log is registered. It could be as such setExtraDataCallback: () => ({ extra: 'data' })

I'll raise a PR to have a more robust example in the documentation.