feat: add dd-trace to edxapp workers

[connorhaugh]

Configuration Pull Request

This PR adds dd-trace to the workers so we can monitor them (and celery tasks). It does not require dd to be enabled.

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.
• Think about how this change will affect Open edX operators and update the wiki page for the next Open edX release if needed

[timmc-edx]

1. I don't think array syntax will work here. I believe what we want is this:
   Suggested change

   	export DD_TAGS=("service:edx-edxapp-{{ SERVICE_VARIANT }}-workers", "queue:{{EDX_REST_API_CLIENT_NAME}}")
   	export DD_TAGS="service:edx-edxapp-{{ SERVICE_VARIANT }}-workers,queue:{{EDX_REST_API_CLIENT_NAME}}"
2. Do we want edx-edxapp-VARIANT-workers, or edx-edxapp-workers-VARIANT? The latter would be a better match for what we have in New Relic.
3. Rather than introducing a secondary use for EDX_REST_API_CLIENT_NAME can we set an environment variable in the workers.conf like EDXAPP_WORKERS_QUEUE? Otherwise we end up yoking those two features together.