* Update Tracer API with instrumentation version
Add option to the `Provider.Tracer` method to specify the
instrumentation version.
Update the global, noop, opentracing bridge, and default SDK
implementations.
This does not propagate the instrumentation library version to the
exported span. That is left for a follow-on PR.
* Revert trace_test.go
This is for the next PR.
* Update SDK to include version for default instrumentation
If the instrumentation library name is empty and the default
instrumentation is uses, include the SDK version.
* Update comments and documentation
* Remove default instrumentation version
* 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)
