Add formatted exception

[EvaSDK]

This is a prototype for adding structured exception to a FluentRecordFormatter formatted message.
I am not sure yet if this is any useful and it is not exactly the cleanest way to add custom attributes to a record but this might be useful for someone.

[EvaSDK]

This is based on MR #18 so the only interesting commit is the last one.

[ngiallelis]

Hi @EvaSDK! We can use logging.exc_text for formatted exception. As I mentioned in #38.

[EvaSDK]

@ngiallelis, well the idea of using fluent is to keep structured log where possible, hence the use of traceback.extract_tb instead of pure text.