Fix observable counter cumulative aggregation

[stormshield-fabs]

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

Changes

The observable counters rely on PrecomputedSum that was using the same ValueMap implementation as Sum (its counterpart for synchronous counters), effectively adding all observations (also drastically reducing the counter range).

This PR specializes ValueMap with an O (operation) parameter so that observations can be performed with the correct semantics (AddAssign for synchronous counters, Assign for observable ones). It also updates the PrecomputedSum::cumulative aggregation to return the current value.

The tests (observable_counter_delta and observable_counter_cumulative) are from #1516 with tiny 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 (none)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: stormshield-fabs (12e0a03)

[codecov]

Codecov Report

Attention: Patch coverage is 99.45652% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 69.3%. Comparing base (f203b03) to head (3036e83).
Report is 24 commits behind head on main.

❗ Current head 3036e83 differs from pull request most recent head 12e0a03. Consider uploading reports for the commit 12e0a03 to get more accurate results

Files	Patch %	Lines
opentelemetry-sdk/src/metrics/internal/sum.rs	90.9%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1644   +/-   ##
=====================================
  Coverage   69.3%   69.3%           
=====================================
  Files        136     136           
  Lines      19637   19554   -83     
=====================================
- Hits       13610   13567   -43     
+ Misses      6027    5987   -40     

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[cijothomas]

• ❌ - login: @stormshield-fabs . The commit (3036e83) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

@stormshield-fabs Could you follow the EasyCLA process and sign it? It is a pre-req for any contributions to OpenTelemetry!

[stormshield-fabs]

The CLA is still under review at my company, thanks for the heads up.

[stormshield-fabs]

The CLA has been signed. I've rebased this PR and updated the changelog.