[log] Make forward iter count optional

[hnyls2002]

related to #9366. Make it optional.

[gemini-code-assist]

Summary of Changes

Hello @hnyls2002, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the system's logging capabilities by introducing a new configuration option to control the display of forward iteration counts in prefill and decode batch logs. It also undertakes a refactoring effort for existing logging-related environment variables, enhancing their organization and providing a clear deprecation path for older settings to ensure a more structured and user-friendly logging setup.

Highlights

• Optional Forward Iteration Logging: Introduced a new environment variable, SGLANG_LOG_FORWARD_ITERS, which allows users to optionally include the iteration count in prefill and decode batch log messages, providing more granular control over log verbosity.
• Logging Environment Variable Refactoring: Consolidated and organized logging-related environment variables under a new 'Logging Options' section in environ.py, improving clarity and maintainability of configuration settings.
• Deprecation of SGLANG_GC_LOG: Renamed the SGLANG_GC_LOG environment variable to SGLANG_LOG_GC and added a deprecation warning to inform users about the change, ensuring a smooth transition for existing configurations.

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[gemini-code-assist]

Code Review

This pull request introduces an option to make the forward iteration count logging optional, controlled by the SGLANG_LOG_FORWARD_ITERS environment variable. This is a useful addition for controlling log verbosity. The changes also include refactoring of logging-related environment variables for better organization, such as renaming SGLANG_GC_LOG to SGLANG_LOG_GC.

My main feedback is to improve the backward compatibility for the renamed environment variable. Currently, it only issues a warning but doesn't use the value from the old variable, which could lead to silent misconfiguration for users. I've suggested a change to handle this gracefully.