celery: allow using links instead of child spans for task execution

[jlc-christie]

Description

As described in #3002, the current default behaviour results in all celery tasks that execute are child spans of the code that pushed it on to the broker, which conflicts with the semantic conventions.

This PR doesn't change the default behaviour since this may be undesirable for consumers expecting the behaviour to be consistent.

Fixes #3002 by providing an optional parameter to the .instrument() method to allow for optionally using span links, e.g.

CeleryInstrumentor().instrument(use_links=True)

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit test added
• Tested E2E locally with jaeger with use_links=False and also use_links=True and observed the expected behaviour: when the flag is False (default) all task executions of a fan-out job are child spans of the code that enqueued it and when True they appeared as linked spans.

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: jlc-christie / name: James Christie (9cc5155, b87b7a0, 99cc8be)

[jlc-christie]

This section of code is just duplicated from test_task, the remaining assertions below are the only meaningful changes