Implements #init_label_set method for all metric classes

[jbernardo95]

This a draft PR of a solution for the problem described in #141.

[coveralls]

Coverage remained the same at 100.0% when pulling 62ab050 on jbernardo95:init-label-set into b3bbe23 on prometheus:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0e0f17f on jbernardo95:init-label-set into 9f08a53 on prometheus:master.

[jbernardo95]

@SuperQ @dmagliola @Sinjo can I get a review on this on when you have the chance?

[dmagliola]

Hey Joao! Sorry for the slow review!
I really like this, thank you for doing it!

I'd like to simplify the code in Histograms a bit (see my comments there), but the rest looks great.

One more important thing, though: Could you add a section to the Readme, probably right before "Reserved labels", briefly explaning the problem and how this method solves it? (you don't want metrics to spontaneously appear the first time they're incremented, you want them to be there from the beginning, so if you know what your labels values are upfront, you can declare them)

And a brief example ideally :)
Thank you

[jbernardo95]

@dmagliola Feedback addressed let me know what you think! 🙂

[dmagliola]

A few requests, mostly around the README, and then we're ready to merge!

[jbernardo95]

So many typos 😬

@dmagliola Ready for a second round 👀

[dmagliola]

2 tiny changes. Please do those two and i'll merge it tomorrow!
Thanks!

[jbernardo95]

Done 🚀

[dmagliola]

Thank you @jbernardo95 for this!