Metrics Service

[panxiaojun233]

What is the purpose of the change

By exposing Dubbo's built-in service, Dubbo-admin can collect data from Metrics statistics.

Brief changelog

XXXXX

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #3702 into master will increase coverage by 0.18%.
The diff coverage is 59.44%.

@@             Coverage Diff              @@
##             master    #3702      +/-   ##
============================================
+ Coverage     63.46%   63.64%   +0.18%     
  Complexity       71       71              
============================================
  Files           703      704       +1     
  Lines         31002    31046      +44     
  Branches       5023     5025       +2     
============================================
+ Hits          19675    19759      +84     
+ Misses         9060     9016      -44     
- Partials       2267     2271       +4

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/org/apache/dubbo/common/Constants.java	93.33% <ø> (ø)	0 <0> (ø)	⬇️
...bo/config/spring/schema/DubboNamespaceHandler.java	100% <100%> (ø)	0 <0> (ø)	⬇️
...in/java/org/apache/dubbo/config/MetricsConfig.java	18.18% <18.18%> (ø)	0 <0> (?)
...in/java/org/apache/dubbo/config/ServiceConfig.java	56.58% <56.58%> (+0.09%)	0 <0> (ø)	⬇️
.../java/org/apache/dubbo/config/ReferenceConfig.java	61.23% <61.23%> (+0.14%)	0 <0> (ø)	⬇️
.../org/apache/dubbo/monitor/dubbo/MetricsFilter.java	80.82% <78.12%> (ø)	0 <0> (?)
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	0% <0%> (ø)
...c/main/java/org/apache/dubbo/rpc/RpcException.java	82.75% <0%> (-3.45%)	0% <0%> (ø)
...bo/rpc/cluster/support/FailbackClusterInvoker.java	67.21% <0%> (-3.28%)	0% <0%> (ø)
...dubbo/remoting/exchange/support/DefaultFuture.java	73.46% <0%> (-1.54%)	0% <0%> (ø)
... and 14 more

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[kezhenxu94]

There are changes that are irrelavant (CRLF and LF) in some files:
ReferenceConfig.java, ServiceConfig.java, and DubboNamespaceHandler.java (and maybe others). @panxiaojun233 could you please revert the irrelavant changes?