Ensure metrics noop types go through constructors

[jtescher]

No description provided.

[codecov]

Codecov Report

Merging #345 into master will decrease coverage by 0.13%.
The diff coverage is 17.24%.

@@            Coverage Diff             @@
##           master     #345      +/-   ##
==========================================
- Coverage   53.83%   53.69%   -0.14%     
==========================================
  Files          69       69              
  Lines        5770     5790      +20     
==========================================
+ Hits         3106     3109       +3     
- Misses       2664     2681      +17     

Impacted Files	Coverage Δ
opentelemetry/src/global/metrics.rs	100.00% <ø> (ø)
opentelemetry/src/api/metrics/noop.rs	20.00% <17.24%> (-2.86%)	⬇️

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