sdk/metric: Reader factories return structs

[pellared]

Fixes #4243

[codecov]

Codecov Report

Merging #4244 (e3467a6) into main (ca2aa83) will increase coverage by 0.0%.
The diff coverage is 95.4%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4244   +/-   ##
=====================================
  Coverage   83.5%   83.5%           
=====================================
  Files        181     181           
  Lines      14161   14161           
=====================================
+ Hits       11825   11827    +2     
+ Misses      2109    2107    -2     
  Partials     227     227           

Impacted Files	Coverage Δ
sdk/metric/periodic_reader.go	91.8% <92.3%> (ø)
sdk/metric/manual_reader.go	85.0% <100.0%> (ø)

... and 1 file with indirect coverage changes