[BugFix] fix decode_preallocated_req_num increase call

[liyonghua0910]

Motivation

修复 a869a06 中的语法错误，该语法错误可能导致 decode 节点重复分配 kv blocks 。

Modifications

main_process_metrics.inc_value("decode_preallocated_req_num")
→ main_process_metrics.decode_preallocated_req_num.inc()

Usage or Command

Accuracy Tests

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.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/2.6@bb122aa). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/engine/common_engine.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##             release/2.6    #8028   +/-   ##
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  Coverage               ?   71.56%           
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  Files                  ?      386           
  Lines                  ?    55740           
  Branches               ?     8753           
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  Hits                   ?    39888           
  Misses                 ?    13031           
  Partials               ?     2821           

Flag	Coverage Δ
GPU	71.56% <0.00%> (?)

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