Add cookbook to expose Prometheus counters.

[lucasdicioccio]

@tchoutri :kissing:

Not sure when I'll have time to address comments, so feel free to directly edit the PR.

The code itself is inspired from what I do in prodapi.

[lucasdicioccio]

@tchoutri any comment?

[tchoutri]

Sorry for the time, I got sidetracked by servant-quickcheck, which needed some love.

Any reason why you didn't use https://hackage.haskell.org/package/wai-middleware-prometheus for the WAI integration? I'm asking since it's part of the same family of packages as wai-middleware-prometheus.

[lucasdicioccio]

Any reason why you didn't use https://hackage.haskell.org/package/wai-middleware-prometheus for the WAI integration? I'm asking since it's part of the same family of packages as wai-middleware-prometheus.

Two things.

• (a) the cost of "writing your own" is low enough (as demonstrated in this example) vs. bundling "an extra dependency"
• (b) sometimes you would really benefit from parametrizing the metrics sets via query-params (e.g., to reduce network load), which is not feasible with the above package).

I haven't discussed (a), which is more of a taste/personal-experience judgment but we could add a mention as well (and maybe link to another cookbook to embed a middleware/serve a Raw Application). For (b) it's a bit motivated in the writing, adding a filtering query-param in prodapi is low on my priority list but it's quite often that I wish I had a way to reduce the prometheus payload at the server-side.

[tchoutri]

@lucasdicioccio cool, thanks for the explanation. :)