fix: Further trim public API on views

[cijothomas]

Leftover cleanup from #2988

View trait itself is not required to be public - no functionality change.
Plus few other leftover nits from PR comments earlier.

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.4%. Comparing base (e123996) to head (3712a18).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/mod.rs	50.0%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2989   +/-   ##
=====================================
  Coverage   81.3%   81.4%           
=====================================
  Files        126     126           
  Lines      24570   24573    +3     
=====================================
+ Hits       19996   20004    +8     
+ Misses      4574    4569    -5     

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