Add /metrics api.

[little-cui]

No description provided.

[codecov-io]

Codecov Report

Merging #125 into master will increase coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   66.05%   66.58%   +0.53%     
==========================================
  Files          18       18              
  Lines        3956     3956              
==========================================
+ Hits         2613     2634      +21     
+ Misses       1176     1155      -21     
  Partials      167      167

Impacted Files	Coverage Δ
server/service/util/dependency.go	54.51% <0%> (+2.66%)	⬆️

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[coveralls]

Coverage increased (+0.5%) to 70.921% when pulling 015638d on little-cui:master into 4d6c65d on ServiceComb:master.