Verify OpenTelemetry 1.0.0 compatibility

[slashmo]

Lots has happened in the OpenTelemetry specification since we defined our top-level Tracing API. Now that their Trace API froze, we should go over it once again to check if our API is compatible.

For 0.2.0, we want to ensure that we're on par with OTel 1.0.0.

OpenTelemetry: Definition of "FeatureFreeze"

Document is no longer accepting new substantial changes, except when such changes are necessary to resolve a fundamental problem or inconsistency in the specification. Editorial changes are accepted. The document and the area it describes are considered feature-complete for the version of the specification the document targets. The document in this status is ready for implementation.

• [ ] #39
• [ ] #45
• [ ] #48