Remove the Flush method from Exemplar

[MrAlias]

As pointed out in this comment, the Flush method is not needed in our implementation. We will delete the value holding the reservoir whenever we would have called Flush.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fecb92e) 82.6% compared to head (92c6ee9) 82.6%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4873     +/-   ##
=======================================
- Coverage   82.6%   82.6%   -0.1%     
=======================================
  Files        232     232             
  Lines      18872   18855     -17     
=======================================
- Hits       15600   15585     -15     
+ Misses      2982    2980      -2     
  Partials     290     290             

Files	Coverage Δ
sdk/metric/internal/exemplar/drop.go	100.0% <ø> (ø)
sdk/metric/internal/exemplar/rand.go	97.6% <ø> (-0.1%)	⬇️
sdk/metric/internal/exemplar/storage.go	100.0% <ø> (ø)

... and 1 file with indirect coverage changes