Suppress non-TTY color output on the process name part of the log

Regular logs are not decorated with colors when non-TTY stdout/stderr is
selected as logging output (c.f. `_use_color` function in `vllm/logger.py`).

However, process-based decoration part in `vllm/utils/system_utils.py`
did not implement color output suppression as the regular logger.

It resulted in excess escape sequences in a redirected output
(e.g. `log.txt` after running `vllm serve ... >log.txt 2>&1`) when:

1.  The environment variable `NO_COLOR` is evaluated to `False` and
2.  The output is non-TTY stdout/stderr.

This commit fixes this issue by implementing the same logic
as the regular logger.

Signed-off-by: Tsukasa OI <floss_llm@irq.a4lg.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: a4lg

vllm/utils/system_utils.py

@@ -175,7 +175,12 @@ def set_process_title(
 
 def _add_prefix(file: TextIO, worker_name: str, pid: int) -> None:
     """Add colored prefix to file output for log decoration."""
-    if envs.NO_COLOR:
+    is_tty = hasattr(file, "isatty") and file.isatty()
+    if (
+        envs.NO_COLOR
+        or envs.VLLM_LOGGING_COLOR == "0"
+        or (envs.VLLM_LOGGING_COLOR != "1" and not is_tty)
+    ):
         prefix = f"({worker_name} pid={pid}) "
     else:
         prefix = f"{CYAN}({worker_name} pid={pid}){RESET} "