Implement SumObserver

[dyladan]

https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/api.md#sumobserver

[lykkin]

@dyladan Looking for something to jump in on, do you think this is a good first task? Feel free to assign this to me, or maybe redirect me if there's somewhere better to start.

[dyladan]

I wouldn't say this is very beginner friendly, but it's not too bad either. Nothing in this is really all that difficult, but it may require some familiarity with the metrics api and sdk. Since I don't know your skill level, I can't really say for sure if it is a good task for you.

If you want some idea of how much work it is, you can look at #1117 which shows the renaming of an instrument to see how many places in the code will need to be modified at a minimum. I would also recommend you look at the general implementation of that instrument since it is itself an observer so it is a good match for this one.

If you want me to assign this to you, then let me know. If you're looking for something a little easier, I'd look at possibly #796, #867, #987, #979, #919,
or #920. #919 and #920 are probably the most beginner friendly.

[lykkin]

Thanks for the suggestions! #1117 makes it look reasonable. I've been climbing around in the golang implementation of metrics, so I'm somewhat familiar with the concepts. I think I'll give it a stab, so go ahead and assign it to me.

Thanks again!

[legendecas]

Closing as the issue is no longer applicable.