fix(di): get pending probes as a copy [backport 3.14] (#14626)

Backport 67da5b5 from #14608 to 3.14.

When retrieving the list of pending probes, we make the registry return
a copy of the collection to avoid mutation during iteration. In the
worst case, Python does not warn that the list has been mutated while
iterating over it, causing the iteration loop to end prematurely without
any sings of errors.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, 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: Gabriele N. Tornetta <[email protected]>

Author: dd-octo-sts[bot]

ddtrace/debugging/_probe/registry.py

@@ -203,7 +203,7 @@ def unregister(self, *probes: Probe) -> List[Probe]:
 
     def get_pending(self, location: str) -> List[Probe]:
         """Get the currently pending probes by location."""
-        return self._pending[location]
+        return self._pending[location].copy()
 
     def __contains__(self, probe: object) -> bool:
         """Check if a probe is in the registry."""

releasenotes/notes/fix-di-copy-pending-probes-be21850231275165.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    dynamic instrumentation: fix an issue that prevented multiple probes on the
+    same location from being instrumented.

tests/debugging/test_debugger.py

@@ -553,13 +553,16 @@ def test_debugger_multiple_function_probes_on_same_lazy_module():
         for i in range(3)
     ]
 
+    sys.modules.pop("tests.submod.stuff", None)
+
     with debugger() as d:
         d.add_probes(*probes)
 
         import tests.submod.stuff  # noqa:F401
 
         assert len(d._probe_registry) == len(probes)
         assert all(_.error_type is None for _ in d._probe_registry.values())
+        assert len(d._probe_registry._pending) == 0
 
 
 # DEV: The following tests are to ensure compatibility with the tracer