OEP/ADR to standardize feature toggle annotations in frontend code

[arbrandes]

Description

Currently, there's no standard way to document feature toggles (a.k.a. environment variables, feature flags, configuration flags) in MFE repositories. This makes it hard to take advantage of automation to pull a list of all existing ones, such as https://github.com/openedx/code-annotations which already does this for edx-platform.

Goals

Write either an OEP or an ADR that standardizes how these toggles should be documented in MFEs. Generally speaking:

• The format should ideally work with https://github.com/openedx/code-annotations (which reportedly works with languages other than python)
• The OEP/ADR should instruct developers to reference all toggles in a central part of the code (for instance, in index.jsx when initializing the MFE)
• The document should contain concrete examples on how to do it
• Ideally, the author of the OEP/ADR will also issue a PR to the README in https://github.com/openedx/frontend-template-application referencing it