ObservableGauge collect data points since previous collection

[fraillt]

Fixes #2213

Changes

Some explanation for implementation:

1. observable/asynchronous instruments ALWAYS report data points since previous collection
2. Temporality affects how measurements are aggregated AND when data points are reported
3. LastValue aggregation always use last measured value, no matter what temporality

• Hence: always use Delta temporality for observable gauge (which uses last value aggregation), in order to report data points from previous collection.

I think there are more issues with other observable instruments, but these will require more sophisticated solution to solve...
We probably need to utilize different value map implementations to make things efficient for all metrics, so #2530 will become essential to further improvements/fixes.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.1%. Comparing base (ad38303) to head (ef66b1b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/pipeline.rs	75.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2618   +/-   ##
=====================================
  Coverage   79.0%   79.1%           
=====================================
  Files        119     119           
  Lines      22534   22555   +21     
=====================================
+ Hits       17822   17842   +20     
- Misses      4712    4713    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[cijothomas]

LGTM. Thank you!

Maybe a changelog for letting users know the issue is fixed for ObservableGauge and still open for other Observables.