add healthz check

[appleboy]

fix #285

[codecov-io]

Codecov Report

Merging #286 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #286      +/-   ##
=========================================
+ Coverage   93.39%   93.4%   +0.01%     
=========================================
  Files          16      16              
  Lines        1226    1229       +3     
=========================================
+ Hits         1145    1148       +3     
  Misses         61      61              
  Partials       20      20

Impacted Files	Coverage Δ
gorush/server.go	100% <100%> (ø)	⬆️

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