fix(core): tracing perf improvements (#8176)

Author: benjamincburns

langchain-core/src/callbacks/manager.ts

@@ -1209,6 +1209,7 @@ export class CallbackManager
       } else {
         callbackManager = inheritableHandlers;
       }
+
       callbackManager = callbackManager.copy(
         Array.isArray(localHandlers)
           ? localHandlers.map(ensureHandler)

langchain-core/src/tracers/tracer_langchain.ts

@@ -192,7 +192,15 @@ export class LangChainTracer
 
   static getTraceableRunTree(): RunTree | undefined {
     try {
-      return getCurrentRunTree();
+      return (
+        // The type cast here provides forward compatibility. Old versions of LangSmith will just
+        // ignore the permitAbsentRunTree arg.
+        (
+          getCurrentRunTree as (
+            permitAbsentRunTree: boolean
+          ) => ReturnType<typeof getCurrentRunTree> | undefined
+        )(true)
+      );
     } catch {
       return undefined;
     }