fix #4251: wrap BackgroundTasks in their own span

[davidgss04]

Description

Fixes #4251

FastAPI background tasks were not properly instrumented. While request spans were created correctly, background tasks executed after the response without their own span. As a result, any spans created inside a background task were incorrectly attached directly to the request span, which had already finished, leading to broken trace hierarchies and inaccurate timing information.

The fix patches BackgroundTask.__call__ to wrap its execution in a dedicated span. This ensures that each background task runs within its own span, correctly parented to the request span, and that any spans created inside the task are properly nested.

The patch is applied only once using a guard (hasattr) to avoid double instrumentation, and the original method is restored during uninstrument_app to prevent side effects.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Testing

Added three regression tests:

• test_background_task_span_parents_inner_spans — verifies that background tasks create a wrapper span and that spans created inside the task are correctly parented

• test_uninstrument_app_restores_background_task_call — ensures the original BackgroundTask.__call__ is restored after uninstrumentation

• test_background_task_span_not_duplicated_on_double_instrument_app — verifies that repeated instrumentation does not create duplicate spans or apply the patch multiple times

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: davidgss04 / name: David Santos (ab9b962, c8d6158, fbcac7a)
• ✅ login: tammy-baylis-swi / name: Tammy Baylis (0407cff)

[tammy-baylis-swi]

Thanks for this! The fix to Background tracing makes sense to me and there is no semconv requirement to add HTTP attributes to that particular span. Metrics generated will be the same.

Please could you also commit fixes from:

• tox -e precommit
• tox -e lint-instrumentation-fastapi

[davidgss04]

Good evening, I have applied the requested precommit and lint-instrumentation-fastapi fixes and updated the PR. Thanks!
Note: I amended and force pushed the original commit instead of making a new one.

[tammy-baylis-swi]

Thanks @davidgss04 ! Forgot to also mention: please add an entry to CHANGELOG.md linking this PR.

[davidgss04]

Good evening @tammy-baylis-swi! I have added the entry to CHANGELOG.md has requested. Thanks!
Note: please ignore the last message I sent you (which I deleted already). Thank you.

[tammy-baylis-swi]

Thanks again! This lgtm. 👍 The maintainers will also have to review this.