Simplifying SDK Configuration: Removing Exporter Registration from Meter Provider

[lalitb]

Fixes # (issue)

Changes

Currently we register MetricExporter(s) to MeterProvider, but it is never used. This registration is un-necessary in current implementation as the life-cycle of MetricExporter is managed by MetricReader as here.

This PR removes the MetricExporter registration from MeterProvider, and let the registration be managed through MetricReader.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1350 (eef91e7) into main (e6fb935) will increase coverage by 0.10%.
The diff coverage is 45.84%.

@@            Coverage Diff             @@
##             main    #1350      +/-   ##
==========================================
+ Coverage   92.01%   92.10%   +0.10%     
==========================================
  Files         219      219              
  Lines        8103     8084      -19     
==========================================
- Hits         7455     7445      -10     
+ Misses        648      639       -9     

Impacted Files	Coverage Δ
sdk/src/metrics/meter_context.cc	42.86% <13.34%> (+1.79%)	⬆️
sdk/src/metrics/meter_provider.cc	75.87% <100.00%> (-1.55%)	⬇️
sdk/test/metrics/async_metric_storage_test.cc	90.48% <100.00%> (-0.43%)	⬇️
sdk/test/metrics/meter_provider_sdk_test.cc	77.78% <100.00%> (-0.60%)	⬇️
sdk/test/metrics/metric_reader_test.cc	86.67% <100.00%> (-0.83%)	⬇️

[esigo]

LGTM
Thanks for the PR.