Batch exporter drops last batch on shutdown

[Nancy-Chauhan]

Bug Report\n\nWhen the Python interpreter shuts down, the batch processor's atexit handler races with the daemon flush thread. The last batch of spans in the buffer is sometimes dropped because the thread is killed before it can flush.\n\n## Steps to reproduce\n\npython\nprocessor = BatchProcessor(export_fn=send, batch_size=100)\nfor span in generate_spans(150): # 100 flush, 50 remain\n processor.add(span)\n# Exit without explicit shutdown — last 50 spans lost\n\n\n## Expected\nAll 150 spans exported.\n\n## Actual\nOnly 100 spans exported. The remaining 50 are lost.\n\n## Root cause\nThe daemon thread is killed before the atexit handler runs.