The point is that the most crucial aspect of logging is being able to filter or put triggers on the logs in pre-existing external logging-solutions without having to modify the frameworks you use.

Important, but I'd hesitate to call it the most crucial aspect... that honour probably goes to 'write messages to a sink'

The stated goal of this log library is to provide a base with common useful functionality, in a way that can be extended to fit more complicated use cases. I'd argue that what you are talking about fits into the 'more complicated' - the log API should definitely allow this, in a way that it can be plugged in without log using code having to change, but I don't believe the functionality needs to be in the base library (at this time).