Move metric definition to the same file as other metrics

[mthenw]

Minor refactor.

[codecov]

Codecov Report

Merging #424 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
- Coverage    63.9%   63.87%   -0.03%     
==========================================
  Files          29       29              
  Lines        1651     1650       -1     
==========================================
- Hits         1055     1054       -1     
  Misses        553      553              
  Partials       43       43

Impacted Files	Coverage Δ
httpapi/config.go	0% <0%> (-8%)	⬇️
httpapi/metrics.go	100% <100%> (ø)	⬆️

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