Add cookbook to expose Prometheus counters.

[lucasdicioccio]

@tchoutri 😗

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]

i blame myself

[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.

[tchoutri]

Okay, quite a big review, I can understand if you'd rather let me handle it. :)

[tchoutri]

Rather "the second tool we use to instrument our program[…]"? The thread is not the metric, right?

[lucasdicioccio]

correct, it's lazy writing

[lucasdicioccio]

Maybe We further instrument our program with a second metrics. This second metrics consist of a simple counter. A background thread will increment the counter every second.

wdyt?

[tchoutri]

yep!

[tchoutri]

Suggested change

	newtype Metrics = Metrics {toLBS :: ByteString}
	newtype Metrics = Metrics {getMetrics :: ByteString}

[lucasdicioccio]

maybe FormattedMetrics then so that getMetrics becomes getFormattedMetrics (if you want to have more explicit types)

[tchoutri]

Yeah sure let's go!

[tchoutri]

Suggested change

	mimeRender _ = toLBS
	mimeRender _ = getMetrics

[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. :)