Docs or examples for creating custom prometheus metrics

[starkers]

Issue Description

Hello, I'm just checking out the middleware (don't want to write my own)..

Typically I'm used to just using the vanilla prometheus promauto (EG promauto.NewCounter() to create custom metrics. However with this library and the examples it is not clear how this is done with this middleware.

I can see severally public functions but honestly I can't figure out how to just do this simple thing.

Is this even possible? If so, I'm happy to PR some docs to echo with an example if someone can explain to me how to access them. (thanks)

Checklist

• [x] Dependencies installed
• [x] No typos
• [x] Searched existing issues and docs

Expected behaviour

Actual behaviour

Steps to reproduce

Working code to debug

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