fix(di): get pending probes as a copy (#14608)

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.

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(cherry picked from commit 67da5b5)

Author: P403n1x87

ddtrace/debugging/_probe/registry.py

@@ -195,7 +195,7 @@ 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

@@ -539,13 +539,16 @@ 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