[Cherry-pick] fix requests & block metrics

[liyonghua0910]

Motivation

修复 num_requests_waiting, num_requests_running 有概率不准确的问题，同时也优化 available_gpu_block_num 的精准性。

Modifications

通过绝对值来上报指标，而不是 inc/dec 的增减计算。

Usage or Command

无

Accuracy Tests

修复后未再反馈 num_requests_waiting 问题。

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!