[trc] Improve usability by getting values from Logger or integrating with it

[hasezoey]

I came across tracing and noticed that flexi_logger has some support for it, so i tried it out, and once it is working it is working "great", but the setup is a little more annoying that should likely be, here some points:

• currently tracing can only be setup via the flexi_logger::trc::setup_tracing function (not combined with something like Logger / LoggerHandle)
• no way to clone a FileLogWriter (for example from a Logger or LoggerHandle)
• no way to clone / get a LogSpecification from a Logger
• a way to integrate those "keep around the return value"(/_keep_alive_handles) on a Logger / LoggerHandle
• EDIT: only one Writer is supported (like only to a file or somewhere else, not std* and file)
• EDIT2: trc implies specfile and async (though i dont see the need to)

TL;DR: clone a Logger's configuration (like printing to various places like stdout/stderr and files) or integrating with it

Please note that i dont have too much experience with tracing itself, as i just came around to find it (and async in general).

Maybe i am also completely misunderstanding on how tracing in flexi_logger or tracing in general is meant to be used

example of current non-tracing Logger / LoggerHandle creation we use

[emabee]

Thanks a lot for the feedback! I fully agree. At least when I wrote the stuff, the API of tracing for subscribers was really hard to use, much more than it should be.
I'll give it another try...