add test case for service discovery process

[chickenlj]

Mainly test cases and comments

[codecov-commenter]

Codecov Report

Merging #8607 (d397780) into 3.0 (3ac0c37) will increase coverage by 0.22%.
The diff coverage is 80.30%.

@@             Coverage Diff              @@
##                3.0    #8607      +/-   ##
============================================
+ Coverage     62.88%   63.11%   +0.22%     
+ Complexity      312      311       -1     
============================================
  Files          1118     1118              
  Lines         46840    46842       +2     
  Branches       7052     7061       +9     
============================================
+ Hits          29457    29562     +105     
+ Misses        14087    13983     -104     
- Partials       3296     3297       +1     

Impacted Files	Coverage Δ
...o/common/url/component/DubboServiceAddressURL.java	67.30% <ø> (-0.62%)	⬇️
.../org/apache/dubbo/metadata/ServiceNameMapping.java	100.00% <ø> (ø)
.../apache/dubbo/registry/client/ServiceInstance.java	50.00% <ø> (ø)
...ta/MetadataServiceURLParamsMetadataCustomizer.java	90.00% <ø> (ø)
.../client/metadata/ServiceInstanceMetadataUtils.java	84.61% <ø> (+2.70%)	⬆️
...registry/client/migration/model/MigrationRule.java	72.43% <ø> (ø)
...e/dubbo/registry/integration/RegistryProtocol.java	59.69% <ø> (ø)
...pache/dubbo/remoting/transport/AbstractClient.java	65.71% <ø> (ø)
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	65.38% <ø> (ø)
...n/java/org/apache/dubbo/metadata/MetadataInfo.java	65.53% <50.00%> (+13.24%)	⬆️
... and 37 more

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