OTLP exemplar support

[shakuzen]

Similar to the exemplar support we added for the Prometheus registry, we can add exemplar support for the OTLP registry. See https://opentelemetry.io/docs/reference/specification/metrics/data-model/#exemplars

[0x006EA1E5]

Hi,

Is there are movement on this?

Thanks

[sonallux]

Hi @shakuzen and @jonatan-ivanov, is someone already working on this? We stumpled across this missing feature of the OTLP micrometer registry and I would like to work on this.

[shakuzen]

I believe @jonatan-ivanov has started work on this, but I'll let him give any update or say if he would like help with it.

[jonatan-ivanov]

@sonallux Thank you for the offer! If you want to work on it, please feel free to do so but there is a catch. :) There are multiple different ways to implement this and I'm right now still exploring them. If you have thoughts on how you would implement this or want to create a draft/prototype, please don't hesitate to share, we are appreciating inputs on this.
It is not guaranteed that we will chose the direction you did but feedback is welcomed. I'm only saying this because I don't want you to be disappointed if you work a lot on a PR and we go to a different direction but it can also happen that we will apply the code you come up with or do something influenced by you. Does this sound fair?
I will be in and out in the next two weeks due to conferences so might not be very responsive in the near future but please feel free to share what you have. Other than this issue, you can also create a room with Tommy (@Tommy Ludwig) and me (@Jonatan Ivanov) on https://slack.micrometer.io if you want to discuss/share anything.

[sonallux]

It is not guaranteed that we will chose the direction you did but feedback is welcomed. I'm only saying this because I don't want you to be disappointed if you work a lot on a PR and we go to a different direction but it can also happen that we will apply the code you come up with or do something influenced by you. Does this sound fair?

@jonatan-ivanov Yes, that is okay for me. I have already created a working prototype (see PR #5486). Please let me know, what you think about this draft.