feat: add support for custom labels in the counter metric

[sralloza]

Add support to custom labels for in the counter metric.

Closes #61

The test passes if you launch it alone, but fails when launching all with this message:
panic: a previously registered descriptor with the same fully-qualified name as...

I don't have enough experience with golang and the source code to fix it.

[depado]

Amazing, thanks for your PR, let me have a look at the testing part, maybe I can figure out what's wrong 😄

[depado]

Alright I think I figured it out. Turns out, unregistering is not sufficient as stated in the library documentation:

// Note that even after unregistering, it will not be possible to
// register a new Collector that is inconsistent with the unregistered
// Collector, e.g. a Collector collecting metrics with the same name but
// a different help string. The rationale here is that the same registry
// instance must only collect consistent metrics throughout its
// lifetime.

This is why when executed individually, the test passes, and why it fails when executed with others.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0f5f022) 100.00% compared to head (d1c63c2) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #62   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          247       257   +10     
=========================================
+ Hits           247       257   +10     

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[depado]

Thanks a lot for your PR 👍
Sorry it took me so long to figure out what was going on in the tests.

[depado]

v1.8.0 was released and includes your feature 👍

[sralloza]

Thanks for the review and for the release!