If the flag is set then annotations are collected and the data are exported
in zipkin or stream. If not you still get the correlation ids, so a purely
log-oriented solution will always have useful data on all requests.
Instrumentation should be able to get by with only 2
interfaces: TraceManager and TraceAccessor (the former is
not needed if you aren't starting a new Span). No explicit
access to thread locals or manipulation of thread context
is required (except locally where necessary).
A Span is enclosed by a Trace (actually a view of the complete
Trace that would be constructed remotely).
