temp: enable DD_DJANGO_INSTRUMENT_MIDDLEWARE

[robrap]

Temporarily enabling middleware for end-of-year
to avoid deployments if an issue arises, and to
not have middleware as a blindspot.

See edx/edx-arch-experiments#879

---

Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.