core[patch]: Support LANGSMITH_TRACING env var (#5587)

* [Core:Tracing] accept LANGSMITH_TRACING

* Rework check

---------

Co-authored-by: jacoblee93 <jacoblee93@gmail.com>

Author: hinthornw

langchain-core/src/callbacks/manager.ts

@@ -984,7 +984,8 @@ export class CallbackManager
       getEnvironmentVariable("LANGCHAIN_VERBOSE") === "true" ||
       options?.verbose;
     const tracingV2Enabled =
-      getEnvironmentVariable("LANGCHAIN_TRACING_V2") === "true";
+      getEnvironmentVariable("LANGCHAIN_TRACING_V2") === "true" ||
+      getEnvironmentVariable("LANGSMITH_TRACING") === "true";
 
     const tracingEnabled =
       tracingV2Enabled ||