Collect and Export metric data before PeriodicMetricReader shutdown.

[lalitb]

Fixes #1858

Changes

As discussed in this issue, PeriodicMetricReader should ensure the metric collection and export is done once before shutdown, for all the data points be uploaded. This is also supported in Java , and discussed in specs - open-telemetry/opentelemetry-specification#2983

The shutdown sequence is:
MeterProvider::Shutdown() -> MeterContext::Shutdown() -> MetricCollector() -> Shutdown() -> MetricReader::Shutdown() -> PeriodicMetricReader::OnShutdown()
-> [[[Perform Collect and Shutdown ]]] -> MetricExporter::Shutdown()

This ensures that the MericExporter instance remains valid while MetricReader is doing collect-and-export during it's shutdown. Once the collect-and-export is completed, the MetricExporter is shutdown.

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 #1860 (ae8e0e4) into main (61bc860) will increase coverage by 0.01%.
The diff coverage is 71.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1860      +/-   ##
==========================================
+ Coverage   85.78%   85.78%   +0.01%     
==========================================
  Files         171      171              
  Lines        5243     5252       +9     
==========================================
+ Hits         4497     4505       +8     
- Misses        746      747       +1     

Impacted Files	Coverage Δ
sdk/src/metrics/metric_reader.cc	75.76% <50.00%> (+0.76%)	⬆️
...metrics/export/periodic_exporting_metric_reader.cc	74.08% <73.08%> (+0.17%)	⬆️
sdk/src/trace/batch_span_processor.cc	91.48% <0.00%> (+0.78%)	⬆️

[esigo]

LGTM
Thanks for the PR :)