chore: allow metadata in tracer

libs/core/langchain_core/tracers/langchain.py

@@ -27,6 +27,8 @@ from langchain_core.tracers.base import BaseTracer
 from langchain_core.tracers.schemas import Run
 
 if TYPE_CHECKING:
+    from collections.abc import Mapping
+
     from langchain_core.messages import BaseMessage
     from langchain_core.outputs import ChatGenerationChunk, GenerationChunk
 
@@ -124,6 +126,8 @@ class LangChainTracer(BaseTracer):
         project_name: str | None = None,
         client: Client | None = None,
         tags: list[str] | None = None,
+        *,
+        metadata: Mapping[str, str] | None = None,
         **kwargs: Any,
     ) -> None:
         """Initialize the LangChain tracer.
@@ -139,6 +143,9 @@ class LangChainTracer(BaseTracer):
             tags: The tags.
 
                 Defaults to an empty list.
+            metadata: Additional metadata to include if it isn't already in the run.
+
+                Defaults to None.
             **kwargs: Additional keyword arguments.
         """
         super().__init__(**kwargs)
@@ -150,6 +157,7 @@ class LangChainTracer(BaseTracer):
         self.tags = tags or []
         self.latest_run: Run | None = None
         self.run_has_token_event_map: dict[str, bool] = {}
+        self.metadata = metadata
 
     def _start_trace(self, run: Run) -> None:
         if self.project_name:
@@ -263,6 +271,7 @@ class LangChainTracer(BaseTracer):
         try:
             run.extra["runtime"] = get_runtime_environment()
             run.tags = self._get_tags(run)
+            _patch_missing_metadata(self, run)
             if run.ls_client is not self.client:
                 run.ls_client = self.client
             run.post()
@@ -398,3 +407,12 @@ class LangChainTracer(BaseTracer):
         """Wait for the given futures to complete."""
         if self.client is not None:
             self.client.flush()
+
+
+def _patch_missing_metadata(self: LangChainTracer, run: Run) -> None:
+    if not self.metadata:
+        return
+    metadata = run.metadata
+    for k, v in self.metadata.items():
+        if k not in metadata:
+            metadata[k] = v