feat(langchain): add last trace id property to CallbackHandler (#1230)

Author: hassiebp

langfuse/langchain/CallbackHandler.py

@@ -63,6 +63,8 @@ def __init__(self, *, public_key: Optional[str] = None) -> None:
         self.prompt_to_parent_run_map: Dict[UUID, Any] = {}
         self.updated_completion_start_time_memo: Set[UUID] = set()
 
+        self.last_trace_id = None
+
     def on_llm_new_token(
         self,
         token: str,
@@ -187,6 +189,8 @@ def on_chain_start(
                     level=span_level,
                 )
 
+            self.last_trace_id = self.runs[run_id].trace_id
+
         except Exception as e:
             langfuse_logger.exception(e)
 
@@ -573,6 +577,8 @@ def __on_llm_action(
             else:
                 self.runs[run_id] = self.client.start_generation(**content)
 
+            self.last_trace_id = self.runs[run_id].trace_id
+
         except Exception as e:
             langfuse_logger.exception(e)