ARCHBOM-1584: refactor monitoring app

[robrap]

Description:

• Add ADR to explain new app organization.
• Apply new app organization to the monitoring Django app.
• Remove middleware ordering checks. This is not a typical Django feature and it is painful when refactoring.
• Add CachedCustomMonitoringMiddleware, CodeOwnerMonitoringMiddleware, and MonitoringMemoryMiddleware to the public API.
• Deprecate above middleware from old location.
• Deprecate methods from modules that were always meant to be used from the public API:
  • accumulate
  • increment
  • set_custom_attribute
  • set_custom_attributes_for_course_key
• Add additional custom attributes for deprecated classes and methods to make them safer to retire.

Note: Some additional details in the CHANGELOG.rst change in the PR.

JIRA:

ARCHBOM-1584

Testing instructions:

• I will test with an edx-platform sandbox once edx-platform PR is ready.

Merge checklist:

• Merge https://github.com/edx/edx-platform/pull/25504
• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPi after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

[robrap]

Reviewer notes to help with the review. Please resolve comments that you are done with. Thank you.

Code coverage is an issue on some moved code that had no code coverage, like MonitoringMemoryMiddleware. I will try to add a smoke test.

[feanil]

Looks great, ship it!

[robrap]

Thanks for your thoughts, @idegtiarov. I'll look into whatever is reasonably quick.

1. Black is hopefully coming in the future to help with code consistency.
2. I was trying to squeeze in a step in the right direction, but there will always be more that can be done, especially for existing code that is being moved around.

[robrap]

@idegtiarov: I did what made sense on this PR, and tried to add explanations.

I still have code coverage I need to add to land this PR.

[idegtiarov]

@robrap Thanks for the answers and clarifications.

[robrap]

@feanil: FYI: I plan on merging, but I wanted to let you know I added these tests for an existing middleware to increase coverage: https://github.com/edx/edx-django-utils/blob/241bb42ae5f511c25ab095018d05966cd509c63d/edx_django_utils/monitoring/tests/test_middleware.py