fix(langchain): handle case when parent span wasn't traced (#3656)

It's possible for the parent span to not have been traced (or have been GCd) so a KeyError would be raised when trying to fetch the span for the parent run_id. Now we defensively `.get()` the parent span instead of subscripting it.

sentry_sdk/integrations/langchain.py

@@ -138,7 +138,7 @@ def _create_span(self, run_id, parent_id, **kwargs):
 
         watched_span = None  # type: Optional[WatchedSpan]
         if parent_id:
-            parent_span = self.span_map[parent_id]  # type: Optional[WatchedSpan]
+            parent_span = self.span_map.get(parent_id)  # type: Optional[WatchedSpan]
             if parent_span:
                 watched_span = WatchedSpan(parent_span.span.start_child(**kwargs))
                 parent_span.children.append(watched_span)