WIP: Stats exporter `init/1`

[deadtrickster]

see #107

@hauleth please try it and let me know if it's enough

[hauleth]

Seems good.

[codecov-io]

Codecov Report

Merging #110 into master will decrease coverage by 0.37%. The diff coverage is 41.17%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
- Coverage   80.62%   80.25%   -0.38%     
==========================================
  Files          34       34              
  Lines         707      714       +7     
==========================================
+ Hits          570      573       +3     
- Misses        137      141       +4

Impacted Files	Coverage Δ
src/oc_stat_exporter.erl	46.66% <41.17%> (-0.71%)	:arrow_down:

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