Annotation supports method name as default resource name

[waveng]

Describe what this PR does / why we need it

通过注解方式时，希望可以不指定资源名，而当前时必须指定的。
指定资源名我认为是一个很麻烦的事情，必须要防止资源名相同，但是当一个项目中有多个相同名称的时，而且是不通的人开发，就有可能出现两者指定的资源名是相同的，所以建议可以不用指定资源名称，默认取当前资源完全限定名，就如 dubbo filter 中一样，这样也许更方便

Does this pull request fix one issue?

Describe how you did it

在不指定的时，默认取当前资源完全限定名

Describe how to verify it

在资源上@SentinelResource , 可在dashboard 中查看资源名称

Special notes for reviews

[CLAassistant]

All committers have signed the CLA.

[sczyh30]

Hi, thanks for your contribution. Please fill the PR description as the PR template described. And please open a related issue then link it in this PR. See contribution guidelines for detail :)

Also please sign the CLA.

[codecov-io]

Codecov Report

Merging #187 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #187   +/-   ##
=========================================
  Coverage     50.29%   50.29%           
  Complexity      820      820           
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  Files           140      140           
  Lines          4772     4772           
  Branches        679      679           
=========================================
  Hits           2400     2400           
  Misses         2083     2083           
  Partials        289      289

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[sczyh30]

Here you can just use MethodUtil.getMethodName(method) to get its method representation, which will cache the method name for better performance.

Suggested change

	buf.append(method.getDeclaringClass().getName())
	return MethodUtil.getMethodName(method);

[sczyh30]

Thanks for contributing. I'll enhance the code later.

[waveng]

Thank you