Update default value for simple exemplar reservoir size

[utpilla]

Spec has provided a default value for SimpleExemplarReservoir size: open-telemetry/opentelemetry-specification#3670

Changes

• Update the default size for SimpleExemplarReservoir to 1.

Merge requirement checklist

• Appropriate CHANGELOG.md files updated for non-trivial changes

[codecov]

Codecov Report

Merging #4803 (7852f81) into main (d0d9133) will increase coverage by 0.05%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4803      +/-   ##
==========================================
+ Coverage   83.46%   83.52%   +0.05%     
==========================================
  Files         292      292              
  Lines       11987    11987              
==========================================
+ Hits        10005    10012       +7     
+ Misses       1982     1975       -7     

Files Changed	Coverage Δ
src/OpenTelemetry/Metrics/MetricPoint.cs	61.79% <ø> (ø)

... and 4 files with indirect coverage changes

[vishweshbankwar]

Add https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry/CHANGELOG.md#:~:text=Experimental%20(pre%2Drelease%20builds%20only)%3A

[utpilla]

I think it's fine to not add this. It should be okay to add this only when introducing a public API change.