[Core] Don't count preempted tokens in prefix cache hit rate

[zhuohan123]

Purpose

Do not count preempted tokens in prefix cache hit rate. See #25780 for details.

Test

Existing test should pass, and now when we run:

vllm bench throughput   --model NousResearch/Hermes-3-Llama-3.1-8B   --dataset-name random   --num-prompts 1000 --input-len 1024 --output-len 1024

We see

INFO 09-26 14:24:45 [loggers.py:127] Engine 000: Avg prompt throughput: 0.0 tokens/s, Avg generation throughput: 7097.7 tokens/s, Running: 260 reqs, Waiting: 475 reqs, GPU KV cache usage: 100.0%, Prefix cache hit rate: 0.2%

Prefix cache hit rate droped from 30% to 0.2%.

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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[gemini-code-assist]

Code Review

This pull request correctly modifies the prefix cache hit rate calculation to exclude tokens from preempted requests. The changes are implemented by adding a preemption counter to requests, which is then used to separate statistics for new and preempted requests. The implementation is clean and directly addresses the issue. The associated refactoring in the scheduler improves code readability. I have reviewed the changes and found no issues.

[zhuohan123]

Note: this line is the only actual change in this file, all other changes are code style change.

[WoosukKwon]

LGTM. Thanks for doing this.