Fix DATADOG_DIAGNOSTICS_ENABLE feature switch in edx-arch-experiments

[timmc-edx]

Version 4.1.0 introduced logging of all span-starts for the first 60 seconds of each worker's lifetime. but I configured prod LMS to not use this functionality. Except the feature switch doesn't actually work, so we've had to downgrade to 4.0.0.

A/C:

• Release a safe version of edx-arch-experiments (either with a fixed toggle, or one that at least defaults to false)
• Figure out how this happened and how to prevent it in the future

[timmc-edx]

I've figured out why we had this problem in this app -- our plugin app implementation loads each app while the IDA's Django settings module is being imported, which means the Django settings haven't loaded yet. (Even the settings from higher in the file aren't available.) For now I'll add a warning in the plugin apps docs but maybe we want to load plugins differently (e.g. delayed until after settings have loaded.)

[timmc-edx]

• Fixing app: fix: Remove early span-start logging; fix loading of Django settings #772
• Docs warning: docs: Add warning about loading Django settings in plugin apps openedx/edx-django-utils#439
• TODO for a systemic fix: Plugin apps load at unexpected time in Django startup (no settings available) openedx/edx-django-utils#438