use promhttp.ContinueOnError to not fail on duplicate metrics

[oliver006]

Fixes #315

[coveralls]

Coverage increased (+0.07%) to 81.379% when pulling f46d1f6 on oh_continue_on_error into 2d7b644 on master.

[codecov]

Codecov Report

Merging #321 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #321     +/-   ##
=========================================
+ Coverage   78.03%   78.14%   +0.1%     
=========================================
  Files           2        2             
  Lines         824      828      +4     
=========================================
+ Hits          643      647      +4     
  Misses        148      148             
  Partials       33       33

Impacted Files	Coverage Δ
exporter.go	88.02% <100%> (+0.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2d7b644...f46d1f6. Read the comment docs.