Introduces a FromContext method on the log.Logger interface that
allows contextual key/value pairs to be attached, e.g. per request,
so that any logger using this API will automatically get the per request
context attached. The proposal makes the traceID available for
contextual logger , if available, and would allow logs originating from
a certain HTTP request to be correlated with traceID.
In addition, when tracing not enabled, skip adding
traceID=00000000000000000000000000000000
to logs.
Fixing a couple bugs, adds some tests and hopefully decrease
lock contention when logging.
Switching from using sync.RWMutex to go-kit SwapLogger.
Fixes bug when creating a new logger from an existing one that
screwed up the keyvals and/or lost the logger name.
Ref #44681
