<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Use mdatagen for the env provider. This switches the package test to be
generated and adds an autogenerated section to the readme.
I've also populated the readme with basic usage information.
<!-- Issue number if applicable -->
#### Link to tracking issue
Fixes
https://github.com/open-telemetry/opentelemetry-collector/issues/12465
Evan Bradley