Add support for anonymous tasks

[naormalca]

Description

This PR continues the work that goatsthatcode did on #1052, due to the fact that the author hasn't responded anymore, we are interested in resolving this issue.

This changes the way spans are initialized for the celery instrumentation hooks and handles the case where Celery tasks are not available in the registry and called anonymously using the signature() or send_task()

Fixes #1002, #609
possibly addresses #784

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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

• Ran changes against reproduced issue in repo linked in issue -> https://github.com/goatsthatcode/opentelemetry-instrumentation-celery-example
• Added Unit Test to cover new conditional branch
• Passed existing test suite

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• 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

[srikanthccv]

The last time I tried running the change, it didn't fix the issue. Is it still the case?

[naormalca]

The last time I tried running the change, it didn't fix the issue. Is it still the case?

the change is fixing the issue, it tested with a unit test and integration tests on my project. what scenario reproduced the issue for you?