Refactor Metrics tests and add more

[cijothomas]

Trying to leverage helper methods as much as possible. Not fully done, but moving in that direction.
Added UpDownCounter tests.

[codecov]

Codecov Report

Attention: Patch coverage is 88.95706% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 71.5%. Comparing base (348ec9e) to head (ef23883).
Report is 17 commits behind head on main.

Files	Patch %	Lines
opentelemetry-sdk/src/metrics/mod.rs	88.9%	18 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1737     +/-   ##
=======================================
+ Coverage   71.0%   71.5%   +0.5%     
=======================================
  Files        135     136      +1     
  Lines      20751   20821     +70     
=======================================
+ Hits       14746   14906    +160     
+ Misses      6005    5915     -90     

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