Issue Description

Type: bug report / question / feature

Describe what happened

The highestCpuUsage System Rule has a confusing behavior.
In 1.7.1 highestCpuUsage meant system cpu usage, now the max between process and system cpu usage, which can be confusing.

Describe what you expected to happen

Due to the fix done in the jdk, maybe Sentinel's custom code could be reverted and/or differentiate process and system cpu usage as separate rules

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

A k8s deployment with some cpu-limit, and a java process being stressed.

Tell us your environment

Sentinel 1.8.0
Kubernetes, multi core server

Anything else we need to know?