chore(metric): Add metric interface

[appleboy]

Signed-off-by: Bo-Yi Wu appleboy.tw@gmail.com

[codecov-commenter]

Codecov Report

Merging #40 (752eba2) into master (fbd8614) will decrease coverage by 0.61%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   87.46%   86.84%   -0.62%     
==========================================
  Files           9       10       +1     
  Lines         351      365      +14     
==========================================
+ Hits          307      317      +10     
- Misses         36       40       +4     
  Partials        8        8              

Flag	Coverage Δ
go-1.13	86.02% <76.47%> (-1.44%)	⬇️
go-1.14	86.84% <76.47%> (+0.23%)	⬆️
go-1.15	86.84% <76.47%> (+0.23%)	⬆️
go-1.16	86.02% <76.47%> (-1.44%)	⬇️
go-1.17	86.02% <76.47%> (-0.59%)	⬇️
ubuntu-latest	86.84% <76.47%> (-0.62%)	⬇️

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Impacted Files	Coverage Δ
options.go	80.43% <20.00%> (-7.38%)	⬇️
consumer.go	90.19% <100.00%> (+0.09%)	⬆️
metric.go	100.00% <100.00%> (ø)

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