Remove comparability requirement for Readers

[MrAlias]

Fix #3085

Instead of tracking the Reader to []view.View pair with a map, create a dedicated viewer type for this. This removes the requirement that Readers are comparable so they can act as a map key.

[codecov]

Codecov Report

Merging #3290 (bc20c7c) into main (ccbc38e) will increase coverage by 0.0%.
The diff coverage is 100.0%.

❗ Current head bc20c7c differs from pull request most recent head 7f685bf. Consider uploading reports for the commit 7f685bf to get more accurate results

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3290   +/-   ##
=====================================
  Coverage   77.9%   77.9%           
=====================================
  Files        164     164           
  Lines      11323   11316    -7     
=====================================
- Hits        8822    8818    -4     
+ Misses      2303    2301    -2     
+ Partials     198     197    -1     

Impacted Files	Coverage Δ
sdk/metric/config.go	100.0% <100.0%> (ø)
sdk/metric/pipeline.go	94.5% <100.0%> (+1.0%)	⬆️
sdk/metric/provider.go	100.0% <100.0%> (ø)

[MrAlias]

Superseded by #3387