[#4132] router support set fallback route as downgrading routing strategy

[chengyouling]

No description provided.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (01e4595) 74.15% compared to head (4feb815) 74.26%.
Report is 4 commits behind head on 2.8.x.

❗ Current head 4feb815 differs from pull request most recent head 9e822bf. Consider uploading reports for the commit 9e822bf to get more accurate results

Files	Patch %	Lines
...b/router/distribute/AbstractRouterDistributor.java	88.46%	0 Missing and 3 partials ⚠️
...rvicecomb/core/provider/consumer/InvokerUtils.java	60.00%	1 Missing and 1 partial ⚠️
...ervicecomb/tracing/zipkin/ZipkinTracingFilter.java	0.00%	2 Missing ⚠️
...rvicecomb/tracing/zipkin/ZipkinTracingHandler.java	75.00%	1 Missing and 1 partial ⚠️
...b/tracing/zipkin/ZipkinConsumerTracingHandler.java	0.00%	1 Missing ⚠️
...b/tracing/zipkin/ZipkinProviderTracingHandler.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              2.8.x    #4133      +/-   ##
============================================
+ Coverage     74.15%   74.26%   +0.11%     
+ Complexity      707      706       -1     
============================================
  Files          1594     1594              
  Lines         40146    40177      +31     
  Branches       3659     3665       +6     
============================================
+ Hits          29769    29839      +70     
+ Misses         8889     8850      -39     
  Partials       1488     1488              

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