Set Falsey (#5783)

Seems natural to try to disable logging by setting `MY_VAR=false` rather
than unsetting (especially once you've already set it in the background)

langchain/callbacks/manager.py

@@ -878,6 +878,16 @@ class AsyncCallbackManager(BaseCallbackManager):
 T = TypeVar("T", CallbackManager, AsyncCallbackManager)
 
 
+def env_var_is_set(env_var: str) -> bool:
+    """Check if an environment variable is set."""
+    return env_var in os.environ and os.environ[env_var] not in (
+        "",
+        "0",
+        "false",
+        "False",
+    )
+
+
 def _configure(
     callback_manager_cls: Type[T],
     inheritable_callbacks: Callbacks = None,
@@ -911,18 +921,17 @@ def _configure(
     wandb_tracer = wandb_tracing_callback_var.get()
     open_ai = openai_callback_var.get()
     tracing_enabled_ = (
-        os.environ.get("LANGCHAIN_TRACING") is not None
+        env_var_is_set("LANGCHAIN_TRACING")
         or tracer is not None
-        or os.environ.get("LANGCHAIN_HANDLER") is not None
+        or env_var_is_set("LANGCHAIN_HANDLER")
     )
     wandb_tracing_enabled_ = (
-        os.environ.get("LANGCHAIN_WANDB_TRACING") is not None
-        or wandb_tracer is not None
+        env_var_is_set("LANGCHAIN_WANDB_TRACING") or wandb_tracer is not None
     )
 
     tracer_v2 = tracing_v2_callback_var.get()
     tracing_v2_enabled_ = (
-        os.environ.get("LANGCHAIN_TRACING_V2") is not None or tracer_v2 is not None
+        env_var_is_set("LANGCHAIN_TRACING_V2") or tracer_v2 is not None
     )
     tracer_session = os.environ.get("LANGCHAIN_SESSION")
     debug = _get_debug()