fix(profiling): clear sample pool after fork [backport 2.15] (#11356)

Backport 9de85e3 from #11350 to 2.15.

Tested with native tests

`crossbeam::ArrayQueue` uses Rust `std::sync::atomic` which doesn't seem
to be consistent across `fork()` calls as `std::mutex` from C++ isn't.
So we need to make sure, an equivalent of `std::mutex::unlock()` happens
after fork.

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[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
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Co-authored-by: Taegyun Kim <[email protected]>

ddtrace/internal/datadog/profiling/dd_wrapper/src/sample_manager.cpp

@@ -74,6 +74,12 @@ void
 Datadog::SampleManager::postfork_child()
 {
     Datadog::Sample::postfork_child();
+    if (sample_pool != nullptr) {
+        // Clear the pool to make sure it's in a consistent state.
+        // Suppose there was a thread that was adding/removing sample from the pool
+        // and the fork happened in the middle of that operation.
+        sample_pool = std::make_unique<SynchronizedSamplePool>(sample_pool_capacity);
+    }
 }
 
 void

releasenotes/notes/profiling-sample-pool-461a108e068dea5b.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    profiling: fixes an issue where the sample pool could deadlock after ``fork()``
+      by clearing it in the child process.
+