Add exemplar support to the OTLP HTTP and gRPC exporters

[dashpole]

Fixes #4899

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dd5d054) 82.6% compared to head (27aa0b7) 82.7%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4900   +/-   ##
=====================================
  Coverage   82.6%   82.7%           
=====================================
  Files        232     232           
  Lines      18870   18916   +46     
=====================================
+ Hits       15603   15649   +46     
  Misses      2977    2977           
  Partials     290     290           

Files	Coverage Δ
...ic/otlpmetricgrpc/internal/transform/metricdata.go	100.0% <100.0%> (ø)
...ic/otlpmetrichttp/internal/transform/metricdata.go	100.0% <100.0%> (ø)