[sdk-metrics] ExperimentalAttribute decorations for experimental exemplar APIs

[CodeBlanch]

Follow-up to #5151

Changes

• Defines OTEL1002 in docs for experimental Exemplar APIs.
• Adds .NET 8 ExperimentalAttribute decoration for experimental Exemplar APIs in SDK.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

Merging #5153 (3d7c414) into main (cd000be) will decrease coverage by 0.18%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5153      +/-   ##
==========================================
- Coverage   83.27%   83.10%   -0.18%     
==========================================
  Files         297      297              
  Lines       12380    12379       -1     
==========================================
- Hits        10310    10288      -22     
- Misses       2070     2091      +21     

Flag	Coverage Δ
unittests	83.10% <ø> (-0.18%)	⬇️

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

Files	Coverage Δ
.../Metrics/Builder/MeterProviderBuilderExtensions.cs	100.00% <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)

... and 9 files with indirect coverage changes