add method which allows adding custom metrics

[amitmtrn]

I've added method which allows adding custom metrics.
I tried to keep the code style although I had to make the options globally available in order to use the same appmetrics instance in addMetric method.

[codecov-io]

Codecov Report

Merging #17 into master will decrease coverage by 4.61%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   91.08%   86.47%   -4.62%     
==========================================
  Files           2        2              
  Lines         157      170      +13     
==========================================
+ Hits          143      147       +4     
- Misses         14       23       +9

Impacted Files	Coverage Δ
lib/appmetrics-prometheus.js	86.39% <40%> (-4.64%)	⬇️

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