fix: Cleanup MetricError and use OTelSdkResult instead

[cijothomas]

Continuing from #2604
In another small follow up, I should be able to remove pub enum MetricError completely!

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 8 lines in your changes missing coverage. Please review.

Project coverage is 81.3%. Comparing base (e9ae9f9) to head (8b076e8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/manual_reader.rs	0.0%	6 Missing ⚠️
.../src/metrics/periodic_reader_with_async_runtime.rs	90.0%	1 Missing ⚠️
...telemetry-sdk/src/testing/metrics/metric_reader.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2905     +/-   ##
=======================================
- Coverage   81.3%   81.3%   -0.1%     
=======================================
  Files        126     126             
  Lines      24143   24156     +13     
=======================================
+ Hits       19639   19649     +10     
- Misses      4504    4507      +3     

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[gruebel]

noice