Oh well... This is a very nifty feature - I thought a lot about implementing it, and finally I found the time to do it.
The Boost Logging Lib v2 is soooo flexible, that I could add profiling to it, without changing the library code (ok - actually I did add one word - virtual - to on_dedicated_thread::write_array).
In other words, it's very easy to find out exactly how much time is spent logging - all of it. For instance, assuming you log using stream' operator<<, you can find out the time spent gathering the message. I've updated the docs.
You can get the latest code from SVN, and see an example.
As far as I know, this is the only logging library that allows for this ;)