fix(tracing) lazy importing asyncio should not reset existing trace context (#9272)

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
5. and there's an existing trace context 
6. 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

- [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

- [ ] Title is accurate
- [ ] All changes are related to the pull request's stated goal
- [ ] Description motivates each change
- [ ] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
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](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

---------

Co-authored-by: Brett Langdon <[email protected]>

Author: joeyzhao2018

ddtrace/_trace/provider.py

@@ -113,7 +113,6 @@ class DefaultContextProvider(BaseContextProvider, DatadogContextMixin):
     def __init__(self):
         # type: () -> None
         super(DefaultContextProvider, self).__init__()
-        _DD_CONTEXTVAR.set(None)
 
     def _has_active_context(self):
         # type: () -> bool

releasenotes/notes/fix-asyncio-context-reset-6283f01dc7d46dc4.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    This fix resolves an issue where importing ``asyncio`` after a trace has already been started will reset the currently active span.

tests/conftest.py

@@ -16,12 +16,14 @@
 from typing import Generator  # noqa:F401
 from typing import Tuple  # noqa:F401
 from unittest import mock
+import warnings
 
 from _pytest.runner import call_and_report
 from _pytest.runner import pytest_runtest_protocol as default_pytest_runtest_protocol
 import pytest
 
 import ddtrace
+from ddtrace._trace.provider import _DD_CONTEXTVAR
 from ddtrace.internal.compat import httplib
 from ddtrace.internal.compat import parse
 from ddtrace.internal.remoteconfig.client import RemoteConfigClient
@@ -67,6 +69,17 @@ def test_spans(tracer):
     container.reset()
 
 
+@pytest.fixture(autouse=True)
+def clear_context_after_every_test():
+    try:
+        yield
+    finally:
+        ctx = _DD_CONTEXTVAR.get()
+        if ctx is not None:
+            warnings.warn(f"Context was not cleared after test, expected None, got {ctx}")
+        _DD_CONTEXTVAR.set(None)
+
+
 @pytest.fixture
 def run_python_code_in_subprocess(tmpdir):
     def _run(code, **kwargs):

tests/contrib/asyncio/test_lazyimport.py

@@ -0,0 +1,18 @@
+import pytest  # noqa: I001
+
+
+@pytest.mark.subprocess()
+def test_lazy_import():
+    import ddtrace.auto  # noqa: F401,I001
+    from ddtrace import tracer  # noqa: I001
+
+    assert tracer.context_provider.active() is None
+    span = tracer.trace("itsatest", service="test", resource="resource", span_type="http")
+    assert tracer.context_provider.active() is span
+
+    # Importing asyncio after starting a trace does not remove the current active span
+    import asyncio  # noqa: F401
+
+    assert tracer.context_provider.active() is span
+    span.finish()
+    assert tracer.context_provider.active() is None