Create doc/unit tests for SDK MeterProvider

[lzchen]

Part of #1219

[codecov]

Codecov Report

Patch coverage: 74.5% and project coverage change: +0.1% 🎉

Comparison is base (2ba8f9f) 49.1% compared to head (4c61ae8) 49.3%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1220     +/-   ##
=======================================
+ Coverage   49.1%   49.3%   +0.1%     
=======================================
  Files        173     174      +1     
  Lines      20937   21043    +106     
=======================================
+ Hits       10301   10380     +79     
- Misses     10636   10663     +27     

Files Changed	Coverage Δ
opentelemetry-api/src/global/metrics.rs	0.0% <0.0%> (ø)
opentelemetry-sdk/src/metrics/pipeline.rs	56.7% <ø> (ø)
...telemetry-sdk/src/testing/metrics/metric_reader.rs	5.2% <5.2%> (ø)
opentelemetry-sdk/src/metrics/meter_provider.rs	82.3% <96.2%> (+18.4%)	⬆️

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