Add Exemplars for 1.6.0 prerelease versions

[utpilla]

Changes

• Mark Exemplars and related APIs public again. They were marked internal in Mark Exemplars internal #4533 before 1.5.0 stable release

Merge requirement checklist

• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Merging #4553 (cc84462) into main (6e73bb9) will decrease coverage by 0.22%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4553      +/-   ##
==========================================
- Coverage   85.42%   85.20%   -0.22%     
==========================================
  Files         320      320              
  Lines       12641    12677      +36     
==========================================
+ Hits        10798    10801       +3     
- Misses       1843     1876      +33     

Impacted Files	Coverage Δ
...elemetry.Exporter.Console/ConsoleMetricExporter.cs	0.00% <ø> (ø)
...tryProtocol/Implementation/MetricItemExtensions.cs	86.59% <ø> (-8.26%)	⬇️
.../Metrics/Builder/MeterProviderBuilderExtensions.cs	83.76% <ø> (ø)
...emetry/Metrics/Exemplar/AlwaysOffExemplarFilter.cs	0.00% <ø> (ø)
...lemetry/Metrics/Exemplar/AlwaysOnExemplarFilter.cs	100.00% <ø> (ø)
src/OpenTelemetry/Metrics/Exemplar/Exemplar.cs	100.00% <ø> (ø)
...metry/Metrics/Exemplar/TraceBasedExemplarFilter.cs	0.00% <ø> (ø)
src/OpenTelemetry/Metrics/MetricPoint.cs	61.79% <ø> (ø)

... and 3 files with indirect coverage changes