[3.0] Optimize dubbo:monitor configuration

[zhangyz-hd]

What is the purpose of the change

当用户需要自己扩展dubbo-monitor实现后，配置有问题

如配置<dubbo:monitor procotol="myMonitor"/>，monitorUrl=null
如配置<dubbo:monitor procotol="myMonitor" address="ip:port"/>，monitorUrl=dubbo://ip:port，变成了dubbo协议？！
如配置<dubbo:monitor address="myMonitor://ip:port"/>，monitorUrl=myMonitor://ip:port，当如果扩展不需要ip:port呢

Brief changelog

优化了org.apache.dubbo.config.utils.ConfigValidationUtils#loadMonitor，可支持如下配置方式

• dubbo.monitor.protocol=registry
• dubbo.monitor.protocol=service-discovery-registry
• dubbo.monitor.protocol=user
• dubbo.monitor.address=user://1.2.3.4:5678?k=v
• dubbo.monitor.protocol=user + dubbo.monitor.address=1.2.3.4:5678

Checklist

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Check if is necessary to patch to Dubbo 3 if you are work on Dubbo 2.7
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Add some description to dubbo-website project if you are requesting to add a feature.
• GitHub Actions works fine on your own branch.
• If this contribution is large, please follow the Software Donation Guide.

[codecov-commenter]

Codecov Report

Merging #8477 (623de6c) into 3.0 (961ce5f) will increase coverage by 0.08%.
The diff coverage is 81.25%.

@@             Coverage Diff              @@
##                3.0    #8477      +/-   ##
============================================
+ Coverage     62.83%   62.92%   +0.08%     
- Complexity      311      312       +1     
============================================
  Files          1112     1112              
  Lines         46633    46638       +5     
  Branches       7016     7019       +3     
============================================
+ Hits          29303    29347      +44     
+ Misses        14067    14034      -33     
+ Partials       3263     3257       -6     

Impacted Files	Coverage Δ
...apache/dubbo/common/constants/CommonConstants.java	100.00% <ø> (ø)
...ache/dubbo/config/utils/ConfigValidationUtils.java	76.48% <81.25%> (+2.16%)	⬆️
.../apache/dubbo/remoting/transport/AbstractPeer.java	63.04% <0.00%> (-6.53%)	⬇️
...he/dubbo/remoting/transport/netty/NettyServer.java	70.17% <0.00%> (-3.51%)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	76.64% <0.00%> (-1.46%)	⬇️
...pache/dubbo/remoting/transport/AbstractServer.java	61.42% <0.00%> (-1.43%)	⬇️
...he/dubbo/registry/multicast/MulticastRegistry.java	66.66% <0.00%> (-0.93%)	⬇️
...apache/dubbo/common/extension/ExtensionLoader.java	79.20% <0.00%> (+0.20%)	⬆️
...in/java/org/apache/dubbo/config/ServiceConfig.java	71.03% <0.00%> (+0.27%)	⬆️
.../dubbo/registry/integration/RegistryDirectory.java	48.83% <0.00%> (+0.33%)	⬆️
... and 10 more

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[guohao]

Set protocol to DUBBO here will cause loading default dubbo monitor when the application didnot have any monitor config.

[guohao]

null should be returned in this scene

[zhangyz-hd]

Thanks, I'll fix it.