Adds Brave 4 ServerTracer integration

[codefromthecrypt]

An oversight in the initial Brave 4 TracerAdapter was how server spans
wire differently than local and client ones. Particularly, they carry
sampling status to prevent downstream trace commands from creating
children when unsampled.

fixes #332

Brave 3's ServerTracer works slightly differently than Brave 3's client
or local tracers. Internally, it uses a ServerSpan which keeps track
of the original sample status. To interop with ServerTracer, use the
following hooks:

Use TracerAdapter.getServerSpan to get one of..

• a real span: when a sampled server request preceded this call
• a noop span: when a server request preceded this call, but was not sampled
• null: if no server request preceded this call

brave4Span = TracerAdapter.getServerSpan(brave4, brave3.serverSpanThreadBinder());
// Since the current server span might be empty, you must check null
if (brave4Span != null) {
  // use or finish the span
}

If you have a reference to a Brave 4 span, and know it represents a server
request, use TracerAdapter.setServerSpan to attach it to Brave 3's thread
binder.

TracerAdapter.setServerSpan(brave4Span.context(), brave3.serverSpanThreadBinder());
brave3.serverTracer().setServerSend(); // for example