Provide accessor to the span implementation

[dashpole]

This is needed if the SDK needs to cast to its underlying type.

Part of open-telemetry/opentelemetry-go#93

@nilebox

[nilebox]

Can you please explain why is this necessary?

[dashpole]

This is needed when attempting to add an OpenCensus span to the OpenTelemetry context using otel.ContextWithSpan(context.Context, otel.Span) In order to do that, we need to be able to get the underlying Span implementation. The full implemnetation would look something like:

// This is the OpenCensus FromContext function, implemented using opentelemetry's SpanFromContext
func (o *otelTracer) FromContext(ctx context.Context) *octrace.Span {
	otSpan := otel.SpanFromContext(ctx)
	return // convert the opentelemetry span into the OC span implementation
}

// This is the OpenCensus NewContext function, implemented using opentelemetry's ContextWithSpan
func (o *otelTracer) NewContext(parent context.Context, s *octrace.Span) context.Context {
        // Below, I need to call s.Internal() in order to have an interface I can cast to my internal type.
        // For example, my internal type could wrap an opentelemetry Span, which I could use in ContextWithSpan
	if myspan, ok := s.Internal().(*myspanimplementation); ok {
		return otel.ContextWithSpan(parent, // get an opentelemetry Span from myspan)
	}
	// The user must have created the octrace Span using a different tracer, and we don't know how to store it.
	return parent
}

[nilebox]

Oh I see that makes sense.