feat: add exporter & prometheus exporter

[proost]

Description

Adding Stats exporter package and prometheus exporter

Related issue(s)

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[maypok86]

Wow, it looks great. I think it's better to merge it a little later, along with the update of otter to v2, along with the redesign of the api, which should no longer change at all.

[maypok86]

@proost I will try to collect the ideas in this issue. It will be very cool to know your opinion.