Celery contrib: read custom parent span id from the task headers

[namoshizun]

What does this pull request do?

In a complex Celery workflow, tasks might be invoked in a chain like this:

<A> ---> <B> ---> <C> ... --> <Z> 

In such setting, because the current Celery instrument only propagates the first caller's traceparent, all downstream tasks share the same traceparent from A. This might be undesirable if every task should be regarded as a subtask of its caller.

For instance, "submit_task_for_computation" initiates "do_heavy_lifting" and waits for completion so that it can immediately rollback a database transaction if the job failed. But instead of correlate do_heavy_lifting with submit_task_for_computation, current instrument produces the following trace:

Whereas we actually hope for this:

This PR is to allow a caller task to set the receiver task's parent span by passing a parent_span_id in the Celery headers.

I have prepared this snippet as a demo.

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• Start Time: 2022-03-29T18:04:21.125+0000

• Duration: 20 min 45 sec

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[basepi]

Looks great! We're fixing a CI failure caused by upstream changes in another library, and then we should be able to get the tests run here and merge this.

Can you add a note to the CHANGELOG?

[basepi]

/test

[basepi]

run elasticsearch-ci/docs