fix: incomplete pkg_resources support [backport 2.5] (#8180)

Backport cc0a798 from #8176 to 2.5.

We fix an incomplete support for pkg_resources. This change covers for
the case where pkg_resources was somehow already imported before
ddtrace.

Follow-up on #8113.

## 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`.
- [x] If change touches code that signs or publishes builds or packages,
or handles credentials of any kind, I've requested a review from
`@DataDog/security-design-and-guidance`.

## 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: Gabriele N. Tornetta <P403n1x87@users.noreply.github.com>

ddtrace/internal/module.py

@@ -238,6 +238,12 @@ def __init__(self):
         # type: () -> None
         self._finding = set()  # type: Set[str]
 
+        # DEV: pkg_resources support to prevent errors such as
+        # NotImplementedError: Can't perform this operation for unregistered
+        pkg_resources = sys.modules.get("pkg_resources")
+        if pkg_resources is not None:
+            pkg_resources.register_loader_type(_ImportHookChainedLoader, pkg_resources.DefaultProvider)
+
     def _add_to_meta_path(self):
         # type: () -> None
         sys.meta_path.insert(0, self)  # type: ignore[arg-type]
@@ -369,9 +375,10 @@ class ModuleWatchdog(BaseModuleWatchdog):
 
     def __init__(self):
         # type: () -> None
+        super().__init__()
+
         self._hook_map = defaultdict(list)  # type: DefaultDict[str, List[ModuleHookType]]
         self._om = None  # type: Optional[Dict[str, ModuleType]]
-        self._finding = set()  # type: Set[str]
         self._pre_exec_module_hooks = []  # type: List[Tuple[PreExecHookCond, PreExecHookType]]
 
     @property

releasenotes/notes/fix-incomplete-pkg_resources-support-58baf819ea8705ef.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fix an incomplete support for pkg_resouces that could have caused an
+    exception on start-up.

tests/internal/test_module.py

@@ -453,15 +453,36 @@ def test_module_watchdog_namespace_import_no_warnings():
     import namespace_test.ns_module  # noqa:F401
 
 
+@pytest.mark.subprocess(ddtrace_run=True, env=dict(NSPATH=str(Path(__file__).parent)))
+def test_module_watchdog_pkg_resources_support():
+    # Test that we can access resource files with pkg_resources without raising
+    # an exception.
+    import os
+    import sys
+
+    sys.path.insert(0, os.getenv("NSPATH"))
+
+    import pkg_resources as p
+
+    p.resource_listdir("namespace_test.ns_module", ".")
+
+
 @pytest.mark.subprocess(
-    ddtrace_run=True,
-    env=dict(
-        PYTHONPATH=os.pathsep.join((str(Path(__file__).parent), os.environ.get("PYTHONPATH", ""))),
-    ),
+    env=dict(NSPATH=str(Path(__file__).parent)),
+    err=lambda _: "Can't perform this operation for unregistered loader type" not in _,
 )
-def test_module_watchdog_pkg_resources_support():
+def test_module_watchdog_pkg_resources_support_already_imported():
     # Test that we can access resource files with pkg_resources without raising
     # an exception.
+    import os
+    import sys
+
+    assert "ddtrace" not in sys.modules
+
+    sys.path.insert(0, os.getenv("NSPATH"))
+
     import pkg_resources as p
 
+    import ddtrace  # noqa
+
     p.resource_listdir("namespace_test.ns_module", ".")