complete lookup method of consul registry and add integration test

[Moriadry-zz]

What is the purpose of the change

complete lookup method of consul registry and add integration test. see #3890

Brief changelog

XXXXX

make lookup method return our immediate provider list, just like what we do in https://github.com/apache/incubator-dubbo/blob/9ce0676908adbb5d073727c47b5a32dda153230e/dubbo-registry/dubbo-registry-zookeeper/src/main/java/org/apache/dubbo/registry/zookeeper/ZookeeperRegistry.java#L195 .

Also check this in ConsulRegistryTest.testLookup()

XXXXX

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[Moriadry-zz]

@beiwei30 Hi Ian, could you help me review this pr? I do some fix in it, I also add a test to increase the code coverage, to ensure everything is fine.

[codecov-io]

Codecov Report

Merging #3906 into master will increase coverage by 0.28%.
The diff coverage is 44.44%.

@@             Coverage Diff              @@
##             master    #3906      +/-   ##
============================================
+ Coverage     63.66%   63.94%   +0.28%     
- Complexity       71       97      +26     
============================================
  Files           712      713       +1     
  Lines         31450    31467      +17     
  Branches       5073     5073              
============================================
+ Hits          20023    20122      +99     
+ Misses         9134     9036      -98     
- Partials       2293     2309      +16

Impacted Files	Coverage Δ	Complexity Δ
...g/apache/dubbo/registry/consul/ConsulRegistry.java	56.29% <44.44%> (+56.29%)	24 <2> (+24)	⬆️
...ng/exchange/support/header/HeartbeatTimerTask.java	73.68% <0%> (-5.27%)	0% <0%> (ø)
...e/dubbo/remoting/transport/netty/NettyChannel.java	57.64% <0%> (-4.71%)	0% <0%> (ø)
.../apache/dubbo/remoting/transport/AbstractPeer.java	67.39% <0%> (-4.35%)	0% <0%> (ø)
...pache/dubbo/registry/support/AbstractRegistry.java	79.69% <0%> (-1.92%)	0% <0%> (ø)
...he/dubbo/registry/multicast/MulticastRegistry.java	69.23% <0%> (-0.46%)	0% <0%> (ø)
.../org/apache/dubbo/monitor/dubbo/MetricsFilter.java	79.62% <0%> (-0.38%)	0% <0%> (ø)
...rialize/protostuff/delegate/TimestampDelegate.java	62.5% <0%> (ø)	0% <0%> (?)
...ommon/serialize/protostuff/utils/WrapperUtils.java	93.54% <0%> (+0.44%)	0% <0%> (ø)	⬇️
...dubbo/remoting/exchange/support/DefaultFuture.java	73.46% <0%> (+0.68%)	0% <0%> (ø)	⬇️
... and 11 more

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[beiwei30]

@beiwei30 Hi Ian, could you help me review this pr? I do some fix in it, I also add a test to increase the code coverage, to ensure everything is fine.

Sorry for the late response. I am on it.

[beiwei30]

pls. check my comments.