add native histogram exemplar support

[fatsheep9146]

fix #1126

[ArthurSens]

not a native histogram expert here, just some small comments :)

[beorn7]

Thanks for doing this. I think this implements the correct algorithm, but there is the huge problem of concurrency. See comments. A also think a more basic approach with a single slice rather than two linked list and a map will be faster in practice and consume less memory. Also see comments.

[beorn7]

I'll try to get to this ASAP, but currently, I'm sick.

[fatsheep9146]

I'll try to get to this ASAP, but currently, I'm sick.

Thanks! Please take good care of yourself and rest well, there is no rush

[kakkoyun]

Nice to have this. Thanks for the contribution 🎉

[beorn7]

Thanks a lot. I finally have done a detailed review. Things are a bit tricky, but I hope I have explained it well enough.

In different news, the doc comment for the ExemplarObserver interface (in file observer.go) needs an update, explaining broadly the behavior for native histograms, which should also include a warning that ObserveWithExemplar isn't lock-free if called on a native histogram with configured exemplars so that it should not be called too frequently.

[fatsheep9146]

@beorn7 The pr is ready for review again.

[beorn7]

Thank you very much. I'm on vacation this week, but this is high on my list once I'm back in action.

[beorn7]

Very cool.

I think all that remains is wordsmithing and some formatting tweaks. See all my pedantic comments. :)

[fatsheep9146]

@beorn7
The commit history seems too long, I can squash to one commit if you think there are no more changes needed.

[beorn7]

No worries, I can just squash during merge.

Or, if you like a more detailed commit structure with more than one commit, but not the 20 we have now, you could rebase on your own.

[beorn7]

One thing fell through the cracks: The doc comment to the ObserveWithExemplar method in histogram.go should contain a warning that it isn't lock-free for native histograms with configured exemplars and therefore should not be called in high-frequency settings.

[beorn7]

Otherwise, this looks good. I'll give @ArthurSens @bwplotka and @kakkoyun a few more days to raise objections.

[fatsheep9146]

@beorn7 I only kept two commits, one for first implementation, the other for reconstitution that you suggested in #1471 (comment)