Returns the underlying trace context for use in Brave apis, or null if this object does not
represent a span.
When a span context is returned from
BraveSpan#context(), there's no ambiguity. It
represents the current span. However, a span context can be in an intermediate state when
extracted from headers. In other words, unwrap might not have a
TraceContext to return.
BraveTracer#extract(Format,Object) can return partial
info. For example, in Amazon Web Services, you may be suggested just a trace ID. In other cases, you
might just inherit baggage or a sampling hint.