Change the Reader.Collect Signature.

[MadVikingGod]

This change only modifies the signature of the Collect method to enable future memory reuse. It is part of the solution to #3047.

This does not include changes to the rest of the pipeline to reduce the review surface area.

[codecov]

Codecov Report

Merging #3732 (4ad6f2d) into main (c5c3c95) will increase coverage by 0.0%.
The diff coverage is 95.8%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3732   +/-   ##
=====================================
  Coverage   81.6%   81.6%           
=====================================
  Files        166     166           
  Lines      12432   12445   +13     
=====================================
+ Hits       10154   10167   +13     
  Misses      2063    2063           
  Partials     215     215           

Impacted Files	Coverage Δ
sdk/metric/reader.go	100.0% <ø> (ø)
sdk/metric/manual_reader.go	85.1% <90.0%> (+0.5%)	⬆️
exporters/prometheus/exporter.go	83.5% <100.0%> (+0.1%)	⬆️
sdk/metric/periodic_reader.go	91.6% <100.0%> (+0.3%)	⬆️