Write created lines when negotiating OpenMetrics

[ArthurSens]

Following up on the Design doc, this PR aims to implement writing created timestamps when using OpenMetrics.

[ArthurSens]

cc @bwplotka @macxamin @TheSpiritXIII

[TheSpiritXIII]

Nice work! Just need that client_model changed upstream really :)

[ArthurSens]

@TheSpiritXIII @bwplotka I opened this PR before I understood what needs to be done to have protobuf working in client_golang. Looking at this now, it seems unnecessary since it is focused on OM's text.

Do we want to proceed anyway? (happy to address the comments if this is still useful)

[bwplotka]

I think this might be useful, especially once we implement _Created TS handling for OM in Prometheus (and to test that). But it's low prio, so feel free to close for now OR finish it by adding second function / option for opt-in created timestamp series (:

[ArthurSens]

Once prometheus/prometheus#12733 gets merged I'll get back to this and start working towards ingesting _created from OM :)

[ArthurSens]

@bwplotka @roidelapluie

I'm finally continuing this PR! Rebased and made printing of created lines optional as requested

[ArthurSens]

I'm now looking at how client_golang can use this, maybe this PR will need to change strategies 🤔

Client golang calls NegotiateWithOpenMetrics to decide which content type to use, and then a new encoder is created according to the chosen format.

https://github.com/prometheus/client_golang/blob/e96fb182c22dee42f79827f9b78b4e283a5f8589/prometheus/promhttp/http.go#L162-L183

If we add another option to NewEncoder, telling that there is yet another OM option we will need another option for content-type negotiation as well 🤔 For example:

	case FmtOpenMetrics_0_0_1, FmtOpenMetrics_1_0_0:
		return encoderCloser{
			encode: func(v *dto.MetricFamily) error {
				_, err := MetricFamilyToOpenMetrics(w, v)
				return err
			},
			close: func() error {
				_, err := FinalizeOpenMetrics(w)
				return err
			},
		}
	case FmtOpenMetrics_with_CT:
		encode: func(v *dto.MetricFamily) error {
				_, err := MetricFamilyToOpenMetrics(w, v, WithCreatedLines())
				return err
			},
			close: func() error {
				_, err := FinalizeOpenMetrics(w)
				return err
			},
		}

And that doesn't sound like a good approach.

[ArthurSens]

Added an extra commit that adds the same ToOpenMetricsOption argument to NewEncoder(), so downstream projects can safely opt-in for the extra created lines

[ArthurSens]

@bwplotka, @TheSpiritXIII @SuperQ

It's been a while, but I'm finally continuing this PR and will work on supporting _created lines in OM Text in client_golang and Prometheus afterward.

WIP PRs:

• Parse created timestamps from OpenMetrics-Text format prometheus#13506
• Optionally print OM created lines client_golang#1408

Do you mind giving reviewing this one again?

EDIT: Just saw the test failures after the rebase, will fix it ASAP

[bwplotka]

Nice, well-done, only a small thing, looking good 👍🏽

[ArthurSens]

Conflicts resolved again 😅

[SuperQ]

Minor nit about some memory allocation, otherwise LGTM.