Use weak reference in langchain instrumentation span map

[wrisa]

For added safety, src/opentelemetry/instrumentation/langchain/span_manager.py -> self.spans: Dict[UUID, _SpanState] = {} use a WeakMap (assuming the span can be safely garbage collected after LangChain no longer has reference to the UUID object) or some kind of LRU cache for bounded memory.

[JWinermaSplunk]

Hi @wrisa, I will look into this.