enhance BraintrustSpanProcessor to support custom exporters #67

Job	Run time
build (3.10, ubuntu-latest, 0)	5m 51s
adk-py (3.10, windows-latest) / test	52s
build (3.10, ubuntu-latest, 1)	7m 48s
build (3.11, windows-latest, 1)	14m 1s
build (3.11, ubuntu-latest, 0)	5m 40s
build (3.12, ubuntu-latest, 0)	5m 52s
build (3.10, windows-latest, 0)	11m 25s
build (3.13, ubuntu-latest, 1)	8m 2s
build (3.11, windows-latest, 0)	9m 5s
build (3.12, windows-latest, 0)	10m 25s
build (3.10, windows-latest, 1)	11m 54s
adk-py (3.12, ubuntu-latest) / test	17s
build (3.13, windows-latest, 0)	10m 4s
build (3.12, ubuntu-latest, 1)	8m 4s
build (3.13, ubuntu-latest, 0)	6m 14s
build (3.13, windows-latest, 1)	12m 37s
build (3.12, windows-latest, 1)	12m 39s
adk-py (3.11, windows-latest) / test	39s
adk-py (3.12, windows-latest) / test	42s
adk-py (3.11, ubuntu-latest) / test	15s
build (3.11, ubuntu-latest, 1)	7m 52s
adk-py (3.10, ubuntu-latest) / test	12s
langchain-py (3.11, ubuntu-latest) / test	19s
langchain-py (3.13, windows-latest) / test	44s
langchain-py (3.13, ubuntu-latest) / test	16s
langchain-py (3.10, windows-latest) / test	55s
langchain-py (3.11, windows-latest) / test	1m 33s
langchain-py (3.12, ubuntu-latest) / test	17s
langchain-py (3.10, ubuntu-latest) / test	19s
langchain-py (3.12, windows-latest) / test	49s
upload-wheel	16s
	2h 35m 58s

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