[log] enable max_log_len trim only when needed (#31482)

Signed-off-by: Andy Xie <andy.xning@gmail.com>

vllm/entrypoints/logger.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import logging
 from collections.abc import Sequence
 
 import torch
@@ -26,6 +27,7 @@ class RequestLogger:
         params: SamplingParams | PoolingParams | BeamSearchParams | None,
         lora_request: LoRARequest | None,
     ) -> None:
+        if logger.isEnabledFor(logging.DEBUG):
             max_log_len = self.max_log_len
             if max_log_len is not None:
                 if prompt is not None: