fix(tracing): lazy importing asyncio should not reset existing trace context [backport 2.8]

[github-actions]

Backport 06b8494 from #9272 to 2.8.

This PR introduces two main changes:

1. Do not reset _DD_CONTEXTVAR when initializing a DefaultContextProvider
2. Explicitly clear _DD_CONTEXTVAR after every test in dd-trace-py CI
   1. We also emit a warning, so pytest will show them to us

Motivation

Currently, when

1. asyncio auto instrumentation is patched
2. and there's an existing trace context
3. and asyncio is imported
   Then the existing trace context will be reset to None.
   The expected behavior is that the existing trace context should not be reset for this kind of lazy imports

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-9272-to-2.8
Commit report: 89937ab
Test service: dd-trace-py

✅ 0 Failed, 1288 Passed, 52 Skipped, 35m 39.67s Total duration (4m 7.15s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-05-21 22:34:38

Comparing candidate commit 89937ab in PR branch backport-9272-to-2.8 with baseline commit 3102897 in branch 2.8.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 201 metrics, 9 unstable metrics.