fix(logging): Include langfuse logger in JSON logging when langfuse callback is used (BerriAI#19162)

When JSON_LOGS is enabled and langfuse is configured as a success/failure callback,
the langfuse logger now receives the JSON formatter. This ensures langfuse SDK
log messages (like 'Item exceeds size limit' warnings) are output as JSON with
proper level information, instead of plain text that log aggregators may
incorrectly classify as errors.

Fixes issue where langfuse warnings appeared as errors in Datadog due to missing
log level in unformatted output.

Co-authored-by: openhands <openhands@all-hands.dev>

Author: neubig

litellm/_logging.py

@@ -133,14 +133,34 @@ def _suppress_loggers():
 ]
 
 
+def _get_loggers_to_initialize():
+    """
+    Get all loggers that should be initialized with the JSON handler.
+
+    Includes third-party integration loggers (like langfuse) if they are
+    configured as callbacks.
+    """
+    import litellm
+
+    loggers = list(ALL_LOGGERS)
+
+    # Add langfuse logger if langfuse is being used as a callback
+    langfuse_callbacks = {"langfuse", "langfuse_otel"}
+    all_callbacks = set(litellm.success_callback + litellm.failure_callback)
+    if langfuse_callbacks & all_callbacks:
+        loggers.append(logging.getLogger("langfuse"))
+
+    return loggers
+
+
 def _initialize_loggers_with_handler(handler: logging.Handler):
     """
     Initialize all loggers with a handler
 
     - Adds a handler to each logger
     - Prevents bubbling to parent/root (critical to prevent duplicate JSON logs)
     """
-    for lg in ALL_LOGGERS:
+    for lg in _get_loggers_to_initialize():
         lg.handlers.clear()  # remove any existing handlers
         lg.addHandler(handler)  # add JSON formatter handler
         lg.propagate = False  # prevent bubbling to parent/root