Use generics to dispatch updates in ValueMap

[stormshield-fabs]

Fixes #1517 (incorrect cumulative aggregation for observable counters).

This is a follow-up to #1992 as I cannot push to the original branch.

It re-introduces the ideas from #1644 : the ValueMap with an O (operation) parameter so that observations can be performed with the correct semantics (Increment for synchronous counters, Assign for observable ones).

I've accounted for the pending review comments on the initial PR and refactored the measure method a bit to limit right-ward drift.

Edit to include the additional gains with this change:

Additionally, the previous implementation of the Observable relied on the assumption that the ValueMap is drained after ever collect. This is an area that could be tweaked in the future. This PR removed that dependency anyway, so this is better future proof.

[codecov]

Codecov Report

Attention: Patch coverage is 90.40000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 74.8%. Comparing base (6a16baf) to head (398fdba).

Files	Patch %	Lines
opentelemetry-sdk/src/metrics/internal/mod.rs	30.0%	7 Missing ⚠️
opentelemetry-sdk/src/metrics/internal/sum.rs	95.0%	4 Missing ⚠️
opentelemetry-sdk/src/metrics/mod.rs	97.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2004     +/-   ##
=======================================
- Coverage   75.1%   74.8%   -0.3%     
=======================================
  Files        122     122             
  Lines      20642   20678     +36     
=======================================
- Hits       15503   15473     -30     
- Misses      5139    5205     +66     

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[cijothomas]

LGTM, Thanks for the extra cleanups.