Clone the contrib repo for every docs dependency install to fix docs build

[NathanielRN]

Description

In the linked issue, it was pointed out that the readTheDocs build is failing with the following error message:

ERROR: Invalid requirement: './opentelemetry-python-contrib/exporter/opentelemetry-exporter-datadog' (from line 9 of docs-requirements.txt)
Hint: It looks like a path. File './opentelemetry-python-contrib/exporter/opentelemetry-exporter-datadog' does not exist.

I've seen this error before and hope that this PR has the solution 😄

Related to: #1352

Solution Explanation

Initially I was hoping to take advantage of pip's --src or --target options to have the repo cloned to the directory I wanted (i.e. ./opentelemtry-python-contrib) but pip does not allow certain options in its requirements.txt file 😕

Next, I read how the readTheDocs configuration file works.

• The submodule option seemed possible but in contradiction to splitting the repos.
• readTheDocs supports using conda enviornment variables but new versions of Conda have an issue that means environment variables are not passed to pip correctly so we again can't do --src

So no luck on readTheDocs.

If we knew the path to the virtual env then we could expect the first -e install and clone the repo at the path <venv_path>/src as in the docs but the build says the venv name is python3.8 -mvirtualenv /home/docs/checkouts/readthedocs.org/user_builds/opentelemetry-python/envs/latest so I don't know what would happen if it went to .... /stable 😕

Finally I decided to just go with cloning the repo every time for these packages. The Contrib repo is 40MB large, and there is a 3GB limit for readTheDocs. We should be fine to merge this, but I think a good follow up would be to remove all the references in docs that need these Contrib packages 😄

But if we merge this at least the build will be fixed 😄

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I wasn't able to get a local build of readTheDocs working... but hopefully we can try this first and I'll follow up with additional fixes if needed 😄

Does This PR Require a Contrib Repo Change?

• 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

[NathanielRN]

An ALTERNATIVE more long-term solution to this is #1465

[ocelotl]

This is a good step forward, thanks @NathanielRN 👍

[aabmass]

LGTM, but I don't think this closes #1352. That seems to be an issue with git tags and RTD not picking up the changes to stable.

Lets remove the "fixes" and see if it actually fixes after

[NathanielRN]

@aabmass Sounds good! I have removed 👍