Clarify SumAggregation on asynchronous counter

[legendecas]

What are you trying to achieve?

In the SDK spec on DefaultAggregation, it suggests that for AsynchronousCounter a default SumAggregation is used for aggregation.

Instrument Kind	Selected Aggregation
Counter	Sum Aggregation
Asynchronous Counter	Sum Aggregation
UpDownCounter	Sum Aggregation
Asynchrounous UpDownCounter	Sum Aggregation
Asynchronous Gauge	Last Value Aggregation
Histogram	Histogram Aggregation

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#default-aggregation

When implementing the SumAggregation for the asynchronous counter, I found that in the SumAggregation:

SumAggregation informs the SDK to collect:
The arithmetic sum of Measurement values.
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#sum-aggregation

Whilst to the asynchronous counter, the Measurement values mean:

The callback function is responsible for reporting the Measurements.
Note: Unlike Counter.Add() which takes the increment/delta value, the callback function reports the absolute value of the counter.
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#asynchronous-counter

That's been said, the Measurement values in asynchronous counters are absolute values and not increments. To SumAggregation, the arithmetic sum of Measurement values of asynchronous counters doesn't seem to be meaningful.

This issue also applies to the Asynchronous UpDownCounter.

[jsuereth]

This is a good callout.

In the Java (early) prototype I actually had two types of measurements: Delta + Cumulative, where Async instruments (and Histogram) provided Cumulative measurements and Sync *Counter provided Delta measurements. This helped clarify what aggregation should do by default when it sees these measurements.

However, one thing that may not be clear, is that Sum-Aggregation is meaningful in the presence of Views. If I configure a View that removes a label from measurements of Async instruments, then I need to "sum" together any measurements I'm "joining" when removing the label.

So a quick table of Instrument => Measurement Type => Default Aggregation:

Instrument	Measurement Type	Default Aggregation
Async* Gauge	Cumulative	Last Value
Async* Counter	Cumulative	Sum
Async* UpDownCounter	Cumulative	Sum
Histogram	Cumulative	Explicit Bucket HIstogram
Counter	Delta	Sum
UpDownCounter	Delta	Sum

So, in all these cases, View can muck with how many measurements are seen. The Aggregation determines what to do, and it's the same regardless of measurement type. Might be worth a more in-depth discussion.

[jmacd]

I thought this topic had been covered in the supplementary guidelines, https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/supplementary-guidelines.md, but I see that no such example is given. I will take this issue to update the guidelines to explicitly cover removal of an attribute from a cumulative counter.