It seems that the linking between the Context/SpanContext/Span is not implemented properly.

The OpenTelemetry spec says that the SpanContext should be encapsulated inside the Span, but the Tracer is able to create a new Span with a specific SpanContext, created outside of the Span.

For example TraceContext::extract returns a SpanContext and Tracer::startAndActivateSpanFromContext receives SpanContext as parameter.

We should follow the spec for the implementation.