Check isSampled parameter in UpdateWithExemplar method

[ImoutoChan]

Fixes #4851

Changes

Check isSampled parameter in UpdateWithExemplar method before each call to ExemplarReservoir!.Offer

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Appropriate CHANGELOG.md files updated for non-trivial changes

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ImoutoChan (eccfe68, 4b6e609, a4bd3dd)

[cijothomas]

@ImoutoChan Thanks! Could you add an entry to the changelog file as this is a bug fix?

[codecov]

Codecov Report

Merging #5004 (a4bd3dd) into main (86a6ba0) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #5004   +/-   ##
=======================================
  Coverage   83.37%   83.38%           
=======================================
  Files         295      295           
  Lines       12339    12343    +4     
=======================================
+ Hits        10288    10292    +4     
  Misses       2051     2051           

Flag	Coverage Δ
unittests	83.38% <0.00%> (+<0.01%)	⬆️

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

Files	Coverage Δ
src/OpenTelemetry/Metrics/MetricPoint.cs	68.47% <0.00%> (-0.54%)	⬇️

... and 5 files with indirect coverage changes

[ImoutoChan]

@cijothomas sure