Add the Drop exemplar Reservoir implementation

[MrAlias]

Part of #559

This PR is split from #4455

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6f96e67) 82.3% compared to head (fe8261a) 82.3%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4850     +/-   ##
=======================================
- Coverage   82.3%   82.3%   -0.1%     
=======================================
  Files        226     227      +1     
  Lines      18557   18563      +6     
=======================================
+ Hits       15288   15292      +4     
- Misses      2981    2983      +2     
  Partials     288     288             

Files	Coverage Δ
sdk/metric/internal/exemplar/drop.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes