chore: flush shared string file before unlocking (#15333)

## Description

We make sure that writes to shared string files are flushed before the
exclusive lock is released to guarantee that changes are visible to
other processes trying to access the same file.

Author: P403n1x87

ddtrace/internal/ipc.py

@@ -181,3 +181,7 @@ def lock_exclusive(self):
             return
         with open_file(self.filename, "r+b") as f, WriteLock(f):
             yield f
+            # Flush before releasing the lock. Here we first release the lock,
+            # then close the file. If a read happens in between these two
+            # operations, the reader might see outdated data.
+            f.flush()