Use OTelSdkResult for metric flush

[cijothomas]

Continuing from #2604

[codecov]

Codecov Report

Attention: Patch coverage is 37.03704% with 17 lines in your changes missing coverage. Please review.

Project coverage is 79.2%. Comparing base (4783b64) to head (38c69ca).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../src/metrics/periodic_reader_with_async_runtime.rs	0.0%	9 Missing ⚠️
opentelemetry-sdk/src/metrics/periodic_reader.rs	57.1%	3 Missing ⚠️
opentelemetry-otlp/src/metric.rs	0.0%	1 Missing ⚠️
...pentelemetry-sdk/src/metrics/in_memory_exporter.rs	0.0%	1 Missing ⚠️
opentelemetry-sdk/src/metrics/manual_reader.rs	0.0%	1 Missing ⚠️
opentelemetry-sdk/src/metrics/pipeline.rs	66.6%	1 Missing ⚠️
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2606     +/-   ##
=======================================
- Coverage   79.2%   79.2%   -0.1%     
=======================================
  Files        118     118             
  Lines      22513   22511      -2     
=======================================
- Hits       17831   17829      -2     
  Misses      4682    4682             

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