Add documentation for supported environment variables

[ocelotl]

More and more environment variables are being supported by this project. It would be a good idea to figure out how to keep them organized and documented in a single place where users can easily find documentation for the supported environment variables.

[aabmass]

The sphinx envvar role seems to work pretty well, I used it for OTEL_RESOURCE_ATTRIBUTES:

opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/resources/__init__.py

Lines 56 to 64 in 2b46d11

	.. envvar:: OTEL_RESOURCE_ATTRIBUTES
	The :envvar:`OTEL_RESOURCE_ATTRIBUTES` environment variable allows resource
	attributes to be passed to the SDK at process invocation. The attributes from
	:envvar:`OTEL_RESOURCE_ATTRIBUTES` are merged with those passed to
	`Resource.create`, meaning :envvar:`OTEL_RESOURCE_ATTRIBUTES` takes *lower*
	priority. Attributes should be in the format ``key1=value1,key2=value2``.
	Additional details are available `in the specification
	<https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/resource/sdk.md#specifying-resource-information-via-an-environment-variable>`_.

And here's how it shows in the docs: https://opentelemetry-python.readthedocs.io/en/latest/sdk/resources.html#envvar-OTEL_RESOURCE_ATTRIBUTES

[codeboten]

This is where they should show up https://opentelemetry-python.readthedocs.io/en/latest/sdk/environment_variables.html, missing docstrings for this though.

[aabmass]

#1680 added docstrings. The remaining work is filling in the docstrings with missing details.

I'm unassigning myself and someone else can flesh those out if I don't get to it

[codeboten]

Renaming this as the work left here is to document supported variables.

[github-actions]

This issue was marked stale due to lack of activity. It will be closed in 30 days.

[rnishtala]

Hello, I would like to work on this and add some documentation to the environment variables.

[lzchen]

Fixed by https://github.com/open-telemetry/opentelemetry-python/pull/1898/files