热点参数并发数限流[BUG]

## Issue Description

Type: *bug report*

### Describe what happened
并发测试热点参数，限流不稳定。
限制并发数为10，并发20次，多次测试，部分通过次数超过10，部分符合预期

### Describe what you expected to happen
并发限制为10

### How to reproduce it (as minimally and precisely as possible)

1. apifox 并发测试
2.
3.

### Tell us your environment
1.8.3

### Anything else we need to know?
ParameterMetric.getThreadCount方法先于addThreadCount调用，同一时刻多个线程进入getThreadCount时获取当前线程数均为null