* 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)
* update always and never sample descriptions
* fix typo
* rename always on / off sampler files, structs and variables to match
Co-authored-by: Rahul Patel <rahulpa@google.com>