Optionally add OM unit

[vesari]

This PR adds support for unit for Open Metrics. It is ready for review, but not ready to merge (see my long comment below). Fixes #684.

[ArthurSens]

Hey @vesari , just passing by to ask if there is anything we could do to unblock you 👋

[vesari]

Hey @vesari , just passing by to ask if there is anything we could do to unblock you 👋

@ArthurSens hello! I think I'm just using a Go version which is too new, I plan on fixing that later today. Thanks a lot for checking on me, much appreciated! :)

[vesari]

This is amazing. Solid work, thanks for finding way to not break compatibility.

Added a few suggestions and ideas, hope those help! Overall this goes in a good direction 👍🏽

Thank you very much, I'm working on addressing your requests for changes and your observations. I'll get back to you once I'm done :)

[vesari]

This is now ready for review :)

[bwplotka]

Early review, looks good!

[bwplotka]

I'd inline this

[bwplotka]

I'd add this inline and show unit is empty? We expanded the data structure so we can expand the error too

[bwplotka]

Info likely does not have unit?

[bwplotka]

As found in https://gist.github.com/bwplotka/a38c33a74fad88a46c4623e924fa58c8#file-prometheusclient_golang1392-md, I think indeed we should handle the case of no unit here gracefully (-1 index response):

> Unsafe Substring Slice during Runtime Metric Unit Extraction

File with exact line number: prometheus/go_collector_latest.go:L213
Link to a file line on GitHub PR: https://github.com/prometheus/client_golang/pull/1392/files#diff-8360f0c0ae2f0d98fb9ecdf37119d6d333dc17ec23f0547055c1ddfdf4ed0759R213
Problem: The unit is extracted using d.Name[strings.IndexRune(d.Name, ':')+1:]. If a runtime metric is introduced without a :, strings.IndexRune evaluates to -1 resulting in -1+1 = 0. The unit silently turns into the entire metric name, bypassing obvious fault detection. (Note: this extraction assumption existed before the PR for histograms, but was generalized here).
Suggestion: Consider checking if strings.ContainsRune(d.Name, ':') before slicing to future-proof the collector against misconfigured runtime metrics.

[vesari]

Ready for review.

[bwplotka]

It looks generally good, but some comments are not address. If you want to skip them, that's ok, just let us know what's the rationales

Thanks!

[bwplotka]

Unrelated change, but if we want this we could simplify further:

Suggested change

	var (
	enc expfmt.Encoder
	encOpts []expfmt.EncoderOption
	)
	if opts.EnableOpenMetricsTextCreatedSamples {
	enc = expfmt.NewEncoder(w, contentType, expfmt.WithCreatedLines())
	} else {
	enc = expfmt.NewEncoder(w, contentType)
	encOpts = append(encOpts, expfmt.WithCreatedLines())
	}
	enc = expfmt.NewEncoder(w, contentType, encOpts...)
	var encOpts []expfmt.EncoderOption
	if opts.EnableOpenMetricsTextCreatedSamples {
	encOpts = append(encOpts, expfmt.WithCreatedLines())
	}
	enc := expfmt.NewEncoder(w, contentType, encOpts...)

[bwplotka]

Let's do in separate PR

[vesari]

I could not comment if not in the form a review as the GitHub UI is being whimsical for some reason and also doesn't allow me to accept your code suggestion

[vesari]

@bwplotka About your comment: "do you plan to address this?"
I had originally addressed your Dec 2 comment asking to skip printing the unit when it's an empty string. But then in your March 9 review on desc.go:232, you asked to show the unit inline even when empty ("I'd add this inline and show unit is empty? We expanded the data structure so we can expand the error too"). The current unit: "" in these test expectations follows that guidance. Maybe I misunderstood something?

[vesari]

@bwplotka about your "ping" comment:
I think I already addressed this? TestHandlerWithEmptyUnit (line 586), TestHandlerWithLongUnit (line 615),and TestHandlerWithEmojiUnit (line 648) are all there. Are they not what you expected?

[bwplotka]

Thanks for catching my misunderstanding (error vs String). All good except one tiny nit!