Cleanup OTLP exporter compression options, add tests

[aabmass]

Description

• Clean up OTLP exporter compression options and envvar code.
• Added tests for this code as well.
• Rename env vars OTEL_EXPORTER_OTLP_SPAN_* -> OTEL_EXPORTER_OTLP_TRACES_* per spec update
• Updated and moved envvar documentation into the opentelemetry.sdk.environment_variables package
• Fixed the changelog formatting 😄 moved to Fixed changelog formatting issues #1679

Relevant part of spec:

Supported values for OTEL_EXPORTER_OTLP_*COMPRESSION options:

• If the value is missing, then compression is disabled.
• gzip is the only specified compression method for now. Other options MAY be supported by language SDKs and should be documented for each particular language.

The gRPC python impl supports Deflate so I have added it as well.

Fixes #1327

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Added lots of new test cases

Does This PR Require a Contrib Repo Change?

Answer the following question based on these examples of changes that would require a Contrib Repo Change:

• The OTel specification has changed which prompted this PR to update the method interfaces of opentelemetry-api/ or opentelemetry-sdk/

• The method interfaces of opentelemetry-instrumentation/ have changed

• The method interfaces of test/util have changed

• Scripts in scripts/ that were copied over to the Contrib repo have changed

• Configuration files that were copied over to the Contrib repo have changed (when consistency between repositories is applicable) such as in

  • pyproject.toml
  • isort.cfg
  • .flake8

• When a new .github/CODEOWNER is added

• Major changes to project information, such as in:

  • README.md
  • CONTRIBUTING.md

• Yes. - Link to PR:

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[lzchen]

@ocelotl
Please review