Add getCpuUsageThreshold() method in SystemRuleManager

[yikangfeng]

Describe what this PR does / why we need it

Open getHighest CpuUsage to send custom alarm when system current limit occurs。

Does this pull request fix one issue?

Fixes #769

Describe how you did it

Adding getHighestCpuUsage Method

Describe how to verify it

Access via SystemRuleManager. getHighestCpuUsage

Special notes for reviews

[codecov-io]

Codecov Report

Merging #770 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #770      +/-   ##
============================================
- Coverage        41%   40.92%   -0.08%     
+ Complexity     1334     1333       -1     
============================================
  Files           301      301              
  Lines          8629     8630       +1     
  Branches       1159     1159              
============================================
- Hits           3538     3532       -6     
- Misses         4665     4671       +6     
- Partials        426      427       +1

Impacted Files	Coverage Δ	Complexity Δ
...a/csp/sentinel/slots/system/SystemRuleManager.java	43.16% <0%> (-0.32%)	7 <0> (ø)
...m/alibaba/csp/sentinel/log/DateFileLogHandler.java	54.54% <0%> (-3.04%)	6% <0%> (-1%)
...tinel/slots/block/flow/param/ParamFlowChecker.java	52.7% <0%> (-2.71%)	28% <0%> (-1%)

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[sczyh30]

And I think if you're going to retrieve the current CPU usage and load of your system, you may get them via:

SystemRuleManager.getCurrentCpuUsage();
SystemRuleManager.getCurrentSystemAvgLoad()

The highestCpuUsage is only the value of the SystemRule.

[sczyh30]

LGTM

[sczyh30]

Thanks for contributing!