Reconsider the context keys

[rakyll]

Based on the conversations at open-telemetry/opentelemetry-specification#948, we may need to reconsider the entries we propagate in the context and whether we will allow two different context keys. I'm filing this issue for visibility because it might be API breaking.

[punya]

Does this issue relate to #1069?

[MrAlias]

@rakyll can you add a bit more context here. We want to make progress on this for the RC release and want to know if these relate to the remote/local span context keys or some other keys.

[rakyll]

The link to the spec issue was incorrect, see open-telemetry/opentelemetry-specification#949 for the earlier discussion.