fix: coerce non-string metadata values instead of dropping them

The v4 migration guide states non-string metadata values are
"automatically coerced to strings," but the implementation drops
them with a warning. This breaks LangGraph integration since
LangGraph injects non-string metadata (langgraph_step=int,
langgraph_triggers=list, langgraph_path=tuple) into RunnableConfig
at runtime.

Coerce non-string values via str() before validation, matching
the documented behavior.

Fixes #1571

Author: nicoyoung

langfuse/_client/propagation.py

@@ -253,6 +253,10 @@ def _propagate_attributes(
         validated_metadata: Dict[str, str] = {}
 
         for key, value in metadata.items():
+            # Coerce non-string values to strings (e.g., LangGraph injects
+            # langgraph_step=int, langgraph_triggers=list, langgraph_path=tuple)
+            if not isinstance(value, str):
+                value = str(value)
             if _validate_string_value(value=value, key=f"metadata.{key}"):
                 validated_metadata[key] = value