Looking forward to this. Hopefully a reviewer can get to this PR soon.

In Akka we have the sourceThread MDC context for the thread which invoked the log function which is an async call and so may be different to thread which is the thread that ends up logging it. I really want to be able to do {{%.15mdc

}}.

I would expect the default value to be a full pattern in its own right and have its own format modifiers. One should be able to do {{%1mdc{a:%2mdc{b:%3mdc

}}}}.