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

[a4lg]

Purpose

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 (i.e. either not set or equals to 0) and
2. The output is non-TTY stdout/stderr.

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

Related: #29017

Test Plan

Sorry, no new pytest test case because:

1. The change is too simple and
2. Testing the log seems trickier than I first initially thought.

On many Linux environment, you can manually test this behavior by running:

# 1.  TTY stdout/stderr (color decorations are acceptable)
vllm serve [OPTIONS]
# 2.  non-TTY stdout/stderr (shall not be decorated with colors)
vllm serve [OPTIONS] |& tee /dev/null

By using the tee command and a pipe (on the second invocation), both stdout/stderr (now redirected as tee's stdout) are now non-TTY and decoration with colors shall be suppressed.

Test Result

With this commit, process decorating logs like (APIServer pid=xxxx) are no longer decorated with colors on non-TTY stdout/stderr.

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[gemini-code-assist]

Code Review

This pull request aims to fix an issue where process log prefixes are colored even when output is redirected to a non-TTY device. The change correctly suppresses color for non-TTY outputs. However, it introduces an inconsistency with the main logging framework by not fully respecting the VLLM_LOGGING_COLOR environment variable. I've suggested a change to align the behavior and ensure user configuration for forcing color on or off is consistently applied.

[ApostaC]

cc @njhill