Initial documentation for environment variables

[aabmass]

Description

Depends on #1671, must be rebased before merge

Live demo https://aabmass.github.io/opentelemetry-python/docs-envvar-demo/exporter/otlp/otlp.html

• Centralized all the sphinx .. envvar:: docs into the API/SDK environment variable modules
• Added .. envvar:: docstrings where they were missing, so they now show in the docs
• Added references to that documentation elsewhere where the envvars are relevant (exporters, BSP, etc.)
• Actually writing the docstrings is todo – i was thinking we could commit this and others can fill in what they know about over time

Part of #1147

Type of change

Please delete options that are not relevant.

• This change requires a documentation update

How Has This Been Tested?

Built docs to see how it all looks. Here are some screenshots:

• 
• 
• Sphinx index builds a full list of environment variables
  

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. (it can be done separate)

Checklist:

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

[owais]

I'm not entirely sure about this. I think a global list of all support env vars is great but I don't think it's strange that a component doesn't document in detail every config env var it supports. For example, jaeger exporter now just lists the env vars it supports without explaining what they do or how to use them (types, values, etc). I realize this is easier for us but I highly doubt it'd be better for end users to jump between pages when all they want is to refer to docs for one component. Is there any easy way we an describe these in the component docs in addition to the global list?

[aabmass]

I realize this is easier for us but I highly doubt it'd be better for end users to jump between pages when all they want is to refer to docs for one component.

I was going off of this comment mostly #1147 (comment). The links go directly to the env var documentation, which seems ok to me. Some of the envars don't apply to a specific piece, e.g. OTEL_EXPORTER_OTLP_TIMEOUT is relevant to span and metric exporters.

Is there any easy way we an describe these in the component docs in addition to the global list?

We could put the env var documentation along with the docs to get this, they still appear in the index the same. I'm not sure if there is a way to sort of expand out the link to copy the documentation inline, I'll look into it

[owais]

Them linking to the other detail page is definitely nicer but I still feel having the docs inline on a page would be so much better if easily doable, otherwise this is fine.

[codeboten]

Thanks this is a nice improvement and sets up a good pattern for documenting env variables.