test(sdk): fix flaky BatchProcessor shutdown test (#5663) (#5675)

In test_shutdown_allows_1_export_to_finish, processor.shutdown()
interrupts the in-progress export via exporter.shutdown(). Depending on
thread scheduling, particularly on Windows and PyPy runners, the worker
thread can terminate before shutdown() returns, causing the intermediate
assertion assert worker_thread.is_alive() is True to fail intermittently.

Fix this by:
- Using time.monotonic() instead of time.time() for elapsed time.
- Removing the unstable intermediate is_alive() is True check.
- Waiting deterministically with worker_thread.join(timeout=1) before
  asserting that the worker thread has terminated.

Fixes #5663

Assisted-by: Gemini 3.8 Flash

Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>

Author: Anurag-M1

opentelemetry-sdk/tests/shared_internal/test_batch_processor.py

@@ -274,15 +274,16 @@ def test_shutdown_allows_1_export_to_finish(self, batch_processor_class, telemet
         processor._batch_processor.emit(telemetry)
         processor._batch_processor.emit(telemetry)
         processor._batch_processor.emit(telemetry)
-        before = time.time()
+        before = time.monotonic()
         processor._batch_processor.shutdown(timeout_millis=3000)
-        # Shutdown does not kill the thread.
-        assert processor._batch_processor._worker_thread.is_alive() is True
 
-        after = time.time()
+        after = time.monotonic()
         assert after - before < 3.3
-        # Thread will naturally finish after a little bit.
-        time.sleep(0.1)
+
+        # The exporter shutdown interrupts the in-progress export. Depending
+        # on thread scheduling, the worker may stop before or shortly after
+        # shutdown() returns.
+        processor._batch_processor._worker_thread.join(timeout=1)
         assert processor._batch_processor._worker_thread.is_alive() is False
         # Expect the second call to be interrupted by shutdown, and the third call to never be made.
         assert exporter.sleep_interrupted is True