Update metrics

[matthsmi]

Renamed stats to metrics.

Also moved all metrics instantiation into init().

[codecov-io]

Codecov Report

Merging #34 into master will increase coverage by 2.6%.
The diff coverage is 55.18%.

@@            Coverage Diff            @@
##           master      #34     +/-   ##
=========================================
+ Coverage   29.18%   31.79%   +2.6%     
=========================================
  Files          17       17             
  Lines        2169     2120     -49     
=========================================
+ Hits          633      674     +41     
+ Misses       1492     1402     -90     
  Partials       44       44

Impacted Files	Coverage Δ
internal/config/chan.go	0% <0%> (ø)	⬆️
internal/methods/http.go	23.3% <0%> (ø)	⬆️
internal/config/helpers.go	42.94% <0%> (ø)	⬆️
internal/config/manager.go	0% <0%> (ø)	⬆️
internal/config/handler.go	16.2% <12.5%> (ø)	⬆️
internal/metrics/metrics.go	70.55% <70.55%> (ø)

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