[Exemplars] Add isSampled check for Histograms

[utpilla]

Changes

• Add isSampled check for Histogram updates so that we call ExemplarReservoir only when the ExemplarFilter samples the measurement
• Update Benchmark.NET package version

Merge requirement checklist

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

[codecov]

Codecov Report

Merging #5016 (ac4915e) into main (9ba8887) will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main    #5016      +/-   ##
==========================================
- Coverage   83.41%   83.39%   -0.02%     
==========================================
  Files         296      296              
  Lines       12377    12377              
==========================================
- Hits        10324    10322       -2     
- Misses       2053     2055       +2     

Flag	Coverage Δ
unittests	83.39% <33.33%> (-0.02%)	⬇️

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

Files	Coverage Δ
src/OpenTelemetry/Metrics/MetricPoint.cs	68.27% <33.33%> (-0.20%)	⬇️

... and 5 files with indirect coverage changes

[cijothomas]

💯 ! Thanks for fixing this! I was discussing last week why the perf was not improving a lot despite the Filter!