fix(llmobs): fix propagation memory usage [backport 2.9] (#9392)

Backport be9936b from #9387 to 2.9.

This PR is a follow up of #9152, and attempts to minimize any added
memory overhead by moving the `llmobs` utility import to inside the
conditional check that `LLMObs` is enabled.

By importing inside the `ddtrace.llmobs.` directory we are implicitly
running the `ddtrace.llmobs.__init__.py` code, which involves
instantiating a `LLMObs` instance. This is likely the largest culprit of
the memory overhead.

Moving the import to only happening if LLMObs is enabled should avoid
that added overhead, given that LLMObs is only running in a select few
customer applications at the moment.

## Checklist

- [x] Change(s) are motivated and described in the PR description
- [x] Testing strategy is described if automated tests are not included
in the PR
- [x] Risks are described (performance impact, potential for breakage,
maintainability)
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed or label `changelog/no-changelog` is set
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/))
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))
- [x] If this PR changes the public interface, I've notified
`@DataDog/apm-tees`.

## Reviewer Checklist

- [x] Title is accurate
- [x] All changes are related to the pull request's stated goal
- [x] Description motivates each change
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- [x] Testing strategy adequately addresses listed risks
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] Release note makes sense to a user of the library
- [x] Author has acknowledged and discussed the performance implications
of this PR as reported in the benchmarks PR comment
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Co-authored-by: Yun Kim <[email protected]>

ddtrace/propagation/http.py

@@ -51,7 +51,6 @@
 from ..internal.sampling import SamplingMechanism
 from ..internal.sampling import validate_sampling_decision
 from ..internal.utils.http import w3c_tracestate_add_p
-from ..llmobs._utils import _inject_llmobs_parent_id
 from ._utils import get_wsgi_header
 
 
@@ -978,6 +977,8 @@ def parent_call():
                 headers[_HTTP_BAGGAGE_PREFIX + key] = span_context._baggage[key]
 
         if config._llmobs_enabled:
+            from ddtrace.llmobs._utils import _inject_llmobs_parent_id
+
             _inject_llmobs_parent_id(span_context)
 
         if PROPAGATION_STYLE_DATADOG in config._propagation_style_inject: