Use whole spancontext for link

[dyladan]

From the spec:

A Link is structurally defined by the following properties:

• SpanContext of the Span to link to.
• Zero or more Attributes further describing the link.

Seems pretty clear to me that the link should expose the whole SpanContext including the trace state.

Originally posted by @dyladan in #36 (comment)

Currently our Link is an object that shares some properties with SpanContext. Should we (1) add the missing properties to Link or should we (2) make it type Link = { spanContext: SpanContext, attributes: Attributes }?

To me, the spec seems to imply 2 is the more "correct" choice, but 1 is the less breaking change.

[blumamir]

I vote for (2).
Seems like the right time to do things more correctly with the cost of minor breaking change.
Also, Links are not in wide use currently so it won't break much.

[dyladan]

I think they're used by at least the GCP exporter so they might be used more than you think.

[dyladan]

I would also vote for 2

[blumamir]

I think they're used by at least the GCP exporter so they might be used more than you think.

I also use them in aws-sdk instrumentation for implementing the messaging systems semantic convention specification for batch receiving.
But it should be a very easy fix.