fix: respect timeout_millis in BatchProcessor.force_flush

Fixes open-telemetry#4568.

Author: chimchim89

opentelemetry-sdk/tests/shared_internal/test_batch_processor.py

@@ -194,13 +194,15 @@ def test_force_flush_returns_true_when_all_exported(
     def test_force_flush_returns_false_when_timeout_exceeded(
         self, batch_processor_class, telemetry
     ):
+        export_started = threading.Event()
         call_count = 0
 
         def slow_export(batch):
             nonlocal call_count
             call_count += 1
-            # Sleep long enough that the deadline is exceeded after first batch.
-            time.sleep(0.2)
+            # Signal that export has started, then sleep long enough for deadline to be exceeded.
+            export_started.set()
+            time.sleep(0.5)
 
         exporter = Mock()
         exporter.export.side_effect = slow_export
@@ -214,7 +216,9 @@ def slow_export(batch):
         )
         for _ in range(50):
             batch_processor._batch_processor.emit(telemetry)
-        # 100ms timeout, each export takes 200ms, so deadline is hit after first batch.
+        # Wait until the worker has started its first export before calling force_flush.
+        export_started.wait()
+        # Timeout is 100ms, each export takes 500ms, so deadline is hit before second batch.
         result = batch_processor.force_flush(timeout_millis=100)
         assert result is False
         # Exporter was called at least once but not for all batches.