API: Propagate tracestate

[mwear]

There was a PR to temporarily remove tracestate. It was opened due to confusion around what context belongs in tracestate, but ultimately never merged. The Context Propagation OTEP provides the necessary layers for the different types of context that users can interact with. It doesn't mention tracestate, and that is because tracestate is for context used by the tracer (SDK) implementation.

As discussed in a recent W3 Distributed Tracing Meeting OpenTelemetry should:

• Propagate tracestate
• Does not need to expose it via API
• May expose it via SDK SPI if there is a use case

[fbogsany]

The spec states:

SpanContext MUST be a final (sealed) class.

SpanContext is specified in the API, and currently contains the tracestate. How should this be hidden from the API but exposed (possibly) in the SDK and propagated?

[mwear]

Those words are from the meeting notes on 2019-09-10. I believe the intent is that trace state is for context needed for a tracing vendor and not for end users. So, it does need to be exposed on the SpanContext so that tracing vendors other SDK implementations can interact with it, but it should not be exposed to end users. This probably means that will be "protected" by a comment in the code, and not a language construct.

[fbogsany]

Fixed in #136