Add some unit test for StatisticNode, ClusterNode and DefaultNodeBuilder class

[cdfive]

Describe what this PR does / why we need it

Add some unit test for StatisticNode, ClusterNode and DefaultNodeBuilder class

Does this pull request fix one issue?

NONE

Describe how you did it

Learning how to use and have a try.

Describe how to verify it

Run mvn test

Special notes for reviews

[sczyh30]

👍

[codecov-io]

Codecov Report

Merging #423 into master will increase coverage by 0.6%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #423     +/-   ##
===========================================
+ Coverage      37.7%   38.31%   +0.6%     
- Complexity      981     1006     +25     
===========================================
  Files           235      235             
  Lines          7407     7407             
  Branches       1014     1014             
===========================================
+ Hits           2793     2838     +45     
+ Misses         4254     4209     -45     
  Partials        360      360

Impacted Files	Coverage Δ	Complexity Δ
...sp/sentinel/slots/statistic/data/MetricBucket.java	100% <0%> (+2.94%)	19% <0%> (+1%)	⬆️
...a/csp/sentinel/slots/statistic/base/LeapArray.java	69.62% <0%> (+7.59%)	25% <0%> (+3%)	⬆️
...p/sentinel/slots/statistic/metric/ArrayMetric.java	78.94% <0%> (+14.91%)	28% <0%> (+7%)	⬆️
...a/com/alibaba/csp/sentinel/node/StatisticNode.java	81.96% <0%> (+16.39%)	28% <0%> (+7%)	⬆️
...ibaba/csp/sentinel/slots/block/BlockException.java	57.69% <0%> (+19.23%)	7% <0%> (+3%)	⬆️
...ava/com/alibaba/csp/sentinel/node/ClusterNode.java	100% <0%> (+27.27%)	8% <0%> (+4%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 303ae86...d5ad044. Read the comment docs.

[sczyh30]

LGTM

[sczyh30]

Cool, thanks for contributing! 👍