[proposal] Refactor aggregation and add exemplars support

[MrAlias]

• Simplify the aggregation package
• Move the internal aggregation logic to its own package: aggregate
• Split the Aggregator interface into an Input and Output function (this is how it is used in the SDK anyways)
• Add an exemplar implementation as the internal/exemplar package (part of Add support for exemplars #559).
• Address Precomputed Sum aggregation should filter on collection #4217

[codecov]

Codecov Report

Merging #4220 (e268cc5) into main (844b107) will decrease coverage by 0.6%.
The diff coverage is 66.3%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4220     +/-   ##
=======================================
- Coverage   83.3%   82.7%   -0.6%     
=======================================
  Files        181     192     +11     
  Lines      13986   14714    +728     
=======================================
+ Hits       11657   12179    +522     
- Misses      2102    2301    +199     
- Partials     227     234      +7     

Impacted Files	Coverage Δ
sdk/metric/internal/aggregate/hist.go	0.0% <0.0%> (ø)
sdk/metric/internal/exemplar/drop.go	0.0% <0.0%> (ø)
sdk/metric/internal/exemplar/hist.go	0.0% <0.0%> (ø)
sdk/metric/internal/exemplar/filter.go	50.0% <50.0%> (ø)
sdk/metric/internal/exemplar/rand.go	69.2% <69.2%> (ø)
sdk/metric/internal/exemplar/measurement.go	74.2% <74.2%> (ø)
sdk/metric/internal/aggregate/aggregate.go	78.0% <78.0%> (ø)
sdk/metric/internal/aggregate/lastval.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/sum.go	100.0% <100.0%> (ø)
sdk/metric/internal/exemplar/fixed.go	100.0% <100.0%> (ø)
... and 1 more

... and 5 files with indirect coverage changes

[MrAlias]

Replace with #4455