Things left out of SpanContext

[codefromthecrypt]

While traceId and spanId were recently added to the SpanContext type, routine properties used including parentSpanId and sampling status were left out.

I can cite code back to at least 2012 that uses these properties as well new and different code such as census which exposes the sampling status.

The impact is that usage of the span context will still remain tracer specific. I'm pretty sure you knew this, but I wanted to explicitly make an issue so folks can know why you decided to leave them out.

PS sampling api is still missing from OT, but we are supposed to interpret SAMPLING_PRIORITY the same as if a sampling api call was made, so I think it is relevant.