Metric SDK: Do not export non-observed attribute sets for async instruments

[dashpole]

Fixes #3605
It addresses this issue by calling delete(s.values, attr) in precomputed aggregators for the value of an attribute set when it is sent. This ensures a given observation is only exported if it is recorded in that collection cycle.

For a simple example of the impact, see the diff in sdk/metric/meter_test.go.

[dashpole]

Additionally, for delta async instruments, omitting an attribute set from a callback in one call makes the next call assume that it is starting over from zero, rather than the previously recorded value

[codecov]

Codecov Report

Merging #4290 (62b6e0e) into main (8b7bffc) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4290     +/-   ##
=======================================
- Coverage   83.5%   83.5%   -0.1%     
=======================================
  Files        183     183             
  Lines      14218   14217      -1     
=======================================
- Hits       11886   11881      -5     
- Misses      2105    2109      +4     
  Partials     227     227             

Impacted Files	Coverage Δ
sdk/metric/meter.go	85.9% <ø> (ø)
sdk/metric/internal/aggregate/sum.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[MrAlias]

Looks good overall, just small questions.

Where do we plan to document the introduced behavior here and our recommendation to use these types of instruments for this feature? opentelemetry.io?

[dashpole]

Where do we plan to document the introduced behavior here and our recommendation to use these types of instruments for this feature? opentelemetry.io?

I'm happy to document it here for now. If we can make sure this behavior is consistent across languages, then I think opentelemetry.io would be great.

[dashpole]

Would the SDK instrumentation constructors be an appropriate place to document the behavior? e.g. https://github.com/open-telemetry/opentelemetry-go/blob/main/sdk/metric/meter.go#L110

[MrAlias]

Would the SDK instrumentation constructors be an appropriate place to document the behavior? e.g. https://github.com/open-telemetry/opentelemetry-go/blob/main/sdk/metric/meter.go#L110

I think there and the RegisterCallback would be good places 👍

[dashpole]

I think there and the RegisterCallback would be good places 👍

Done