Add a getContext -> SpanContext in the Span

[bogdandrutu]

the SpanContext contains the ids that uniquely identify a Span (trace_id, span_id) and the propagated trace options.

Every Span class should have it's own SpanContext:

class Span {
  SpanContext getContext() {
    return my_own_context;
  }
}

We should also remove the context() from the Tracer because that was the incoming SpanContext but when we make a call to a different user we should not use that context we actually should use the SpanContext of the current Span.

=======================   // Client in JS
    Sent.RequestSpanA       // SpanA
=======================
           | here we should have the SpanContext of the "SpanA" that way "SpanB" is a child of "SpanA"
=======================   // Service in PHP
    Recv.RequestSpanB       // SpanB
=======================
    Sent.RequestSpanC       // SpanC
=======================
           | here we should have the SpanContext of the "SpanC" that way "SpanD" is a child of "SpanC"
=======================  // DB in Java
    Recv.RequestSpanA       // SpanD
=======================

If I would have to write an instrumentation using the current API in PHP when I make the call to the DB I would probably propagate the RequestTracer.context(); which actually is the SpanContext of the "SpanA".

[chingor13]

In the current implementation, RequestTracer::context() is always the current context. This is because the context is managed separately, depending on whether the extension is available or not. If the extension is available, then the extension is responsible for managing the span context. If not, then we manage it in PHP using the static (per request) variables.