Feature spring cloud zuul adapter

[tigerMoon]

Describe what this PR does / why we need it

Spring cloud zuul lack the capability of flow control. this starter implements this.

Does this pull request fix one issue?

Fixes #63

Describe how you did it

Implement ZuulFilter at pre, post, and error phase. provide ServiceId and API PATH level flow control.

Describe how to verify it

1. run test cases.
2. include starter in local spring cloud project.

Special notes for reviews

no

[codecov-io]

Codecov Report

Merging #188 into master will increase coverage by 1.18%.
The diff coverage is 64.51%.

@@             Coverage Diff              @@
##             master     #188      +/-   ##
============================================
+ Coverage     37.05%   38.24%   +1.18%     
- Complexity      940      992      +52     
============================================
  Files           224      235      +11     
  Lines          7221     7407     +186     
  Branches        984     1014      +30     
============================================
+ Hits           2676     2833     +157     
+ Misses         4207     4205       -2     
- Partials        338      369      +31

Impacted Files	Coverage Δ	Complexity Δ
...tinel/adapter/zuul/fallback/DefaultUrlCleaner.java	0% <0%> (ø)	0 <0> (?)
...er/zuul/fallback/DefaultBlockFallbackProvider.java	100% <100%> (ø)	4 <4> (?)
...pter/zuul/fallback/DefaultRequestOriginParser.java	100% <100%> (ø)	2 <2> (?)
...dapter/zuul/properties/SentinelZuulProperties.java	45.45% <45.45%> (ø)	3 <3> (?)
.../sentinel/adapter/zuul/fallback/BlockResponse.java	46.66% <46.66%> (ø)	3 <3> (?)
...inel/adapter/zuul/filters/SentinelErrorFilter.java	56.25% <56.25%> (ø)	5 <5> (?)
...aba/csp/sentinel/adapter/zuul/util/FilterUtil.java	59.67% <59.67%> (ø)	12 <12> (?)
...tinel/adapter/zuul/filters/SentinelPostFilter.java	62.5% <62.5%> (ø)	3 <3> (?)
...dapter/zuul/fallback/ZuulBlockFallbackManager.java	71.42% <71.42%> (ø)	4 <4> (?)
...l/adapter/zuul/filters/AbstractSentinelFilter.java	83.33% <83.33%> (ø)	2 <2> (?)
... and 15 more

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[sczyh30]

Thanks for contributing. I'll do the review these days.

[tigerMoon]

this pr has decoupled sentinel zuul adapter from Spring Cloud Zuul. the Spring Cloud Zuul adapter will put into spring cloud alibaba project and dependant with this adapter.

[junjiex123]

这个adapter什么时候可以搞定？

[sczyh30]

Implement ZuulFilter at pre, post, and error phase. provide ServiceId and API PATH level flow control.

这里有个问题哈：有没有可能出现以下 entry/exit 匹配不上的情况（按时间线）：

1. 请求 1 进入 SentinelPreFilter，调用 SphU.entry(res1)
2. 请求 2 进入 SentinelPreFilter（与之前同一线程），调用 SphU.entry(res2)
3. 请求 1 完成，从 SentinelPostFilter 处调 entry.exit()（与之前同一线程或不同线程），此时 currentContext 的 entry 可能不是 entry1，exit 的时候不匹配会抛出异常

entry 与 exit 需要成对出现，匹配调用，否则调用链会不正确，不匹配会抛出异常。

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Is it possible that resource entry/exit in SentinelPreFilter/SentinelPostFilter does not match, which causes error?

[tigerMoon]

Implement ZuulFilter at pre, post, and error phase. provide ServiceId and API PATH level flow control.

这里有个问题哈：有没有可能出现以下 entry/exit 匹配不上的情况（按时间线）：

1. 请求 1 进入 SentinelPreFilter，调用 SphU.entry(res1)
2. 请求 2 进入 SentinelPreFilter（与之前同一线程），调用 SphU.entry(res2)
3. 请求 1 完成，从 SentinelPostFilter 处调 entry.exit()（与之前同一线程或不同线程），此时 currentContext 的 entry 可能不是 entry1，exit 的时候不匹配会抛出异常

entry 与 exit 需要成对出现，匹配调用，否则调用链会不正确，不匹配会抛出异常。

Is it possible that resource entry/exit in SentinelPreFilter/SentinelPostFilter does not match, which causes error?

不会出现entry/exit 匹配不上的情况。
Zuul 使用的单线程单请求的模型，SphU.entry(res1) 之后，success 之后会走SentinelPostFilter， exit.
抛异常会走ErrorFilter, exit.

[sczyh30]

LGTM

[sczyh30]

Awesome, thanks for contributing!

[sczyh30]

@tigerMoon Could you please add Chinese document here: https://github.com/sentinel-group/sentinel-website/blob/master/docs/zh-cn/open-source-framework-integrations.md

We'll merge this with wiki later.