fix(span_processor): guard span_formatter debug call behind isEnabledFor check

The  method evaluates  inside an f-string
passed to , which causes full span serialization
on every span end, regardless of the effective log level. This is a
measurable per-span CPU cost.

Because Python f-strings are evaluated before being handed to the
logger, the formatting runs unconditionally even when DEBUG logging is
disabled.

Guard the debug call with
so the expensive  only runs when DEBUG level is
actually enabled.

Resolves: langfuse/langfuse#15339

Author: Konstantin Khlopkov

langfuse/_client/span_processor.py

@@ -12,6 +12,7 @@
 """
 
 import base64
+import logging
 import os
 import threading
 from typing import Callable, Dict, List, Optional, cast
@@ -218,9 +219,10 @@ def on_end(self, span: ReadableSpan) -> None:
                 )
                 return
 
-            langfuse_logger.debug(
-                f"Trace: Processing span name='{span._name}' | Full details:\n{span_formatter(span)}"
-            )
+            if langfuse_logger.isEnabledFor(logging.DEBUG):
+                langfuse_logger.debug(
+                    f"Trace: Processing span name='{span._name}' | Full details:\n{span_formatter(span)}"
+                )
 
             super().on_end(span)
         finally: