Fix Asynchronous Counters Recording

[MrAlias]

Fix #3278

Currently, asynchronous counters observe values that are treated as incremental changes to already summed recordings. This is contrary to what the specification notes is expected by the user in the API:

Note: Unlike Counter.Add() which takes the increment/delta value, the callback function reports the absolute value of the counter. To determine the reported rate the counter is changing, the difference between successive measurements is used.

This fixes the implemented behavior. It adds a new precomputedSum Aggregator type that interprets the passed Aggregate values as already computed sums and returns them directly. This new Aggregator type is used for the AsyncCounter and AsyncUpDownCounter instrument kinds.

[codecov]

Codecov Report

Merging #3350 (144a05b) into main (2d02a2f) will increase coverage by 0.0%.
The diff coverage is 92.8%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3350   +/-   ##
=====================================
  Coverage   77.9%   77.9%           
=====================================
  Files        164     164           
  Lines      11319   11345   +26     
=====================================
+ Hits        8821    8847   +26     
  Misses      2301    2301           
  Partials     197     197           

Impacted Files	Coverage Δ
sdk/metric/pipeline.go	93.4% <85.7%> (-0.5%)	⬇️
sdk/metric/internal/sum.go	100.0% <100.0%> (ø)
exporters/jaeger/jaeger.go	91.1% <0.0%> (+0.8%)	⬆️

[MadVikingGod]

This seems reasonable.

Just for my understanding does this basically mean that async Counters are gauges with monotonic flags and temporality fields?

[MrAlias]

Just for my understanding does this basically mean that async Counters are gauges with monotonic flags and temporality fields?

I think the AsyncUpDownCounter could be though of this abstractly. It is still exported as a Sum data type, by default, in the end though.

The way I understand the specification to be written is that the user of asynchronous up-down-counters and counters maintain the state of the counter themselves. It is passed to the SDK with the appropriate monotonic and temporality types to have the SDK report the sum with the right properties, but it is up to the user to ensure the validity of these properties.