bthread_creation_max_latency延迟过大

**Describe the bug (描述bug)**
非BUG，问题定位。
开启了FLAGS_show_bthread_creation_in_vars，目前显示bthread创建时间（max延迟）过大，max达到100-150ms，P9999看起来最大在10ms，正常不到1ms，但是本身bthread usage使用率不到一半，CPU/内存负载并不高，请问有什么情况下会导致这个bthread创建时间过大？

worker usage:
```
bthread_timer_usage : 0.00517015
bthread_worker_count : 45
bthread_worker_usage : 15.9525
bthread_worker_usage_51840 : 0.380111
bthread_worker_usage_51841 : 0.254136
bthread_worker_usage_51842 : 0.371774
bthread_worker_usage_51843 : 0.251471
bthread_worker_usage_51844 : 0.24348
bthread_worker_usage_51845 : 0.301761
bthread_worker_usage_51846 : 0.254467
bthread_worker_usage_51847 : 0.256903
bthread_worker_usage_51848 : 0.2545
bthread_worker_usage_51849 : 0.324174
bthread_worker_usage_51850 : 0.382035
bthread_worker_usage_51851 : 0.254862
bthread_worker_usage_51852 : 0.25711
bthread_worker_usage_51853 : 0.250151
bthread_worker_usage_51854 : 0.249609
bthread_worker_usage_51855 : 0.372599
bthread_worker_usage_51856 : 0.375154
bthread_worker_usage_51857 : 0.254612
bthread_worker_usage_51858 : 0.25015
bthread_worker_usage_51859 : 0.377874
bthread_worker_usage_51860 : 0.250818
bthread_worker_usage_51861 : 0.375357
bthread_worker_usage_51862 : 0.253064
bthread_worker_usage_51863 : 0.302341
bthread_worker_usage_51864 : 0.297399
bthread_worker_usage_51865 : 0.372837
bthread_worker_usage_51866 : 0.241416
bthread_worker_usage_51867 : 0.367837
bthread_worker_usage_51868 : 0.378367
bthread_worker_usage_51869 : 0.378046
bthread_worker_usage_51870 : 0.378745
bthread_worker_usage_51871 : 0.305297
bthread_worker_usage_51872 : 0.379981
bthread_worker_usage_51873 : 0.325682
bthread_worker_usage_51874 : 0.304342
bthread_worker_usage_51875 : 0.372135
bthread_worker_usage_51876 : 0.390805
bthread_worker_usage_51877 : 0.379916
bthread_worker_usage_51878 : 0.252689
bthread_worker_usage_51879 : 0.406393
bthread_worker_usage_51880 : 0.295973
bthread_worker_usage_51881 : 0.306851
bthread_worker_usage_51882 : 0.234954
bthread_worker_usage_51883 : 0.302123
bthread_worker_usage_51884 : 0.248518
```
**To Reproduce (复现方法)**

**Expected behavior (期望行为)**

**Versions (各种版本)**
OS:
Compiler:
brpc:
protobuf:

**Additional context/screenshots (更多上下文/截图)**

No source file or test is named. Start by tracing the bthread creation latency reported when FLAGS_show_bthread_creation_in_vars is enabled, using the supplied worker metrics and reported max/P9999 values to investigate the discrepancy. Done means identifying the conditions or cause, or narrowing the issue into a reproducible, actionable follow-up.

Written by the indexing model from the issue text.