[sdk-metrics] Improve exemplar tests

[CodeBlanch]

[Originally part of #5364]

Changes

• Adds internal ExemplarReservoir factory on view API.
• Use the view API to test delta/cumulative temporality behaviors in exemplar tests.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated

[codecov]

Codecov Report

Attention: Patch coverage is 90.32258% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 83.14%. Comparing base (6250307) to head (6f3070a).
Report is 101 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5393      +/-   ##
==========================================
- Coverage   83.38%   83.14%   -0.25%     
==========================================
  Files         297      284      -13     
  Lines       12531    12070     -461     
==========================================
- Hits        10449    10035     -414     
+ Misses       2082     2035      -47     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	83.11% <90.32%> (?)
unittests-Solution-Stable	83.12% <90.32%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/OpenTelemetry/Metrics/AggregatorStore.cs	80.48% <100.00%> (+0.09%)	⬆️
src/OpenTelemetry/Metrics/Metric.cs	96.77% <100.00%> (+0.25%)	⬆️
src/OpenTelemetry/Metrics/MetricReaderExt.cs	91.50% <100.00%> (+1.50%)	⬆️
...OpenTelemetry/Metrics/MetricStreamConfiguration.cs	80.76% <100.00%> (+5.76%)	⬆️
src/OpenTelemetry/Metrics/MetricPoint.cs	66.60% <62.50%> (-1.88%)	⬇️

... and 52 files with indirect coverage changes

[cijothomas]

LGTM.
Thanks for the cumulative testing targeting the spec question! open-telemetry/opentelemetry-specification#3891
Very helpful.