logging should not use global state/variables

[BlackHC]

hierarchicalLoggingEnabled, recordStackTraceAtLevel, _rootLevel (and also the _loggers static field) are all global which makes testing code that uses global loggers difficult.

A solution could be to use expandos that are keyed to a zone environment variable for the 'global' logging state. This would make it easy to decouple tests.

[natebosch]

I worry that making these variables, which are already confusing and hard to reason about, zone level concepts we are just making things harder for ourselves.

See #441

If anything I think we should consider an entire overhaul of the API which doesn't suffer from these global impacts. Within that design we should certainly consider whether having some sort of zone scoped log variable would be useful - that's what we do in package:build and it has been working well for us. https://pub.dev/documentation/build/latest/build/log.html

Closing for now since I don't think we plan on addressing this with the current design.