fix(celery): account for a chains scenario

[wantsui]

We've made a few changes to handle celery context recently:

• fix(celery): close celery.apply spans even without after_task_publish, when using apply_async #10676
• fix(celery): handle upstream celery patch and propagation during an e… #11272

In particular the goal of #10676 was to handle a scenario where a long running task may run into an exception, preventing it from closing.

Unfortunately, this scenario did not account for cases where tasks are chained and may not close until later.

See: #11479

With this PR, the sample app in #11479 would attach the celery specific span back to the root span.

This is a draft PR for now because I am waiting to see if there are unexpected failures if we remove the logic to close unclosed prerun spans in some cases. (Some more testing on this is needed).

I also need to add tests for the chains scenario.

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[github-actions]

CODEOWNERS have been resolved as:

ddtrace/contrib/internal/celery/app.py                                  @DataDog/apm-core-python @DataDog/apm-idm-python
ddtrace/contrib/internal/celery/signals.py                              @DataDog/apm-core-python @DataDog/apm-idm-python