[DUBBO-3137]: step3 - start to use ClusterConstants and MonitorConstants

[beiwei30]

What is the purpose of the change

issue #3137: step3 - start to use ClusterConstants and MonitorConstants

Brief changelog

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Verifying this change

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[codecov-io]

Codecov Report

Merging #4045 into master will decrease coverage by 0.63%.
The diff coverage is 72.72%.

@@             Coverage Diff              @@
##             master    #4045      +/-   ##
============================================
- Coverage     63.11%   62.47%   -0.64%     
  Complexity      564      564              
============================================
  Files           753      774      +21     
  Lines         32411    34254    +1843     
  Branches       5143     5384     +241     
============================================
+ Hits          20455    21399     +944     
- Misses         9604    10393     +789     
- Partials       2352     2462     +110

Impacted Files	Coverage Δ	Complexity Δ
...in/java/org/apache/dubbo/config/MetricsConfig.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...c/main/java/org/apache/dubbo/common/Constants.java	75% <ø> (ø)	0 <0> (ø)	⬇️
...g/apache/dubbo/config/AbstractReferenceConfig.java	94% <ø> (ø)	0 <0> (ø)	⬇️
...e/dubbo/registry/integration/RegistryProtocol.java	82.69% <ø> (ø)	0 <0> (ø)	⬇️
.../java/org/apache/dubbo/config/ReferenceConfig.java	59.85% <0%> (ø)	0 <0> (ø)	⬇️
...g/apache/dubbo/config/AbstractInterfaceConfig.java	72% <0%> (ø)	0 <0> (ø)	⬇️
...in/java/org/apache/dubbo/config/ServiceConfig.java	58.2% <0%> (ø)	0 <0> (ø)	⬇️
...dubbo/rpc/cluster/directory/AbstractDirectory.java	96.96% <100%> (ø)	0 <0> (ø)	⬇️
...che/dubbo/registry/dubbo/DubboRegistryFactory.java	89.74% <100%> (ø)	6 <0> (ø)	⬇️
.../rpc/cluster/router/mock/MockInvokersSelector.java	94.44% <100%> (ø)	0 <0> (ø)	⬇️
... and 53 more

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[CrazyHZM]

LGTM