Make PeriodicReader with own Thread the default

[cijothomas]

Metrics/PeriodicReader part of #2386

1. Makes PeriodicReaderWithOwnThread as the default PeriodicReader
2. The existing PeriodicReader that required runtime is now enabled under flag "experimental_metrics_periodicreader_with_async_runtime". Existing functionality retained as-is, just under a feature flag and module name.
3. Modified examples to use the default PeriodicReader.
4. Otlp-Basic-Http does not yet work with PeriodicReader as reqwest client it uses needs tokio runtime. This example is retained as-is, demonstrating that existing functionality can be retained as-is by adding "experimental_metrics_periodicreader_with_async_runtime" feature.

[codecov]

Codecov Report

Attention: Patch coverage is 84.42136% with 105 lines in your changes missing coverage. Please review.

Project coverage is 79.2%. Comparing base (d67d1fc) to head (eedc07e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../src/metrics/periodic_reader_with_async_runtime.rs	76.3%	63 Missing ⚠️
opentelemetry-sdk/src/metrics/periodic_reader.rs	89.7%	41 Missing ⚠️
opentelemetry-sdk/src/metrics/mod.rs	87.5%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2403     +/-   ##
=======================================
- Coverage   79.4%   79.2%   -0.3%     
=======================================
  Files        123     123             
  Lines      21527   21527             
=======================================
- Hits       17111   17065     -46     
- Misses      4416    4462     +46     

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[cijothomas]

Merging to make progress, happy to address feedbacks post merger.