Move Aggregator interface to aggregator package

[bogdandrutu]

Updates #2273

Signed-off-by: Bogdan Drutu [email protected]

[bogdandrutu]

@MrAlias I think it is ok to skip changelog for this since it is not released and is part of the same entry I added in the initial PR.

[codecov]

Codecov Report

Merging #2444 (a10775f) into main (4702f6f) will increase coverage by 0.0%.
The diff coverage is 91.3%.

@@          Coverage Diff          @@
##            main   #2444   +/-   ##
=====================================
  Coverage   76.3%   76.3%           
=====================================
  Files        173     173           
  Lines      12010   12010           
=====================================
+ Hits        9167    9174    +7     
+ Misses      2598    2593    -5     
+ Partials     245     243    -2     

Impacted Files	Coverage Δ
sdk/metric/processor/basic/basic.go	96.9% <ø> (ø)
sdk/metric/aggregator/aggregatortest/test.go	81.0% <60.0%> (ø)
sdk/metric/aggregator/aggregator.go	100.0% <100.0%> (ø)
sdk/metric/aggregator/histogram/histogram.go	97.4% <100.0%> (ø)
sdk/metric/aggregator/lastvalue/lastvalue.go	87.7% <100.0%> (ø)
sdk/metric/aggregator/sum/sum.go	89.6% <100.0%> (ø)
sdk/metric/export/metric.go	100.0% <100.0%> (ø)
sdk/metric/processor/processortest/test.go	84.1% <100.0%> (ø)
sdk/metric/sdk.go	81.5% <100.0%> (+1.4%)	⬆️
sdk/metric/selector/simple/simple.go	100.0% <100.0%> (ø)
... and 2 more