* Update License header for all source files
- Add Apache 2.0 header to source files that did not have one.
- Update all existing headers dated to 2019 to be 2020
- Remove comma from License header to comply with the Apache 2.0
guidelines.
* Update Copyright notice
Use the standard Copyright notices outlined by the
[CNCF](https://github.com/cncf/foundation/blob/master/copyright-notices.md#copyright-notices)
* Add inspectable test tracer
This adds a test Tracer implementation that tracks its active and ended
spans and uses a Span implementation that can be inspected (e.g., to see
what attributes have been set).
* Ensure test tracer can start spans concurrently
* Flip conditional logic to return early
* Remove duplicate test
* Fix file name casing
🤦
* Add comments to testtrace code
* Remove Link and AddLink methods from test Span
* Enable concurrently setting and getting test attrs
* Remove SetAttribute from test tracer
* Fix test
* Fix names post-rebase
