Major Description
The 4.0.0 is much better than 3.x API but a few holes. Here is an umbrella issue to keep track of API improvements to do for htrace 5.0
detach and reattach should return this. Ditto on the annotations.
curThreadTracer should be currentThreadTracer or getCurrentThreadTracer
Have to do too much checking for null when using the 4.0 API; e.g. checking if tracer is null.
In 3.x, there was an isTracing but now you have to get Span from Scope and if it is null, then you know you are not tracing... not as obvious as isTracing.
TraceScope is meant as the Tracing Interface; it hides Spans. Its coverage is not complete though. For example, to add a Parent, you have to get the Span from the Scope and add the Parent there.