Fix PeriodicReader shutdown to invoke shutdown on exporter

[cijothomas]

Was a miss in the PeriodicReader implementation that uses own thread. This was already handled in the old periodic reader.

More refactoring is expected in this area, but this addresses the gap.

[cijothomas]

in a future PR, I expect to replace this with InMemoryExporter itself.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.

Project coverage is 77.2%. Comparing base (36f9caf) to head (8432c18).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/periodic_reader.rs	88.8%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2477   +/-   ##
=====================================
  Coverage   77.1%   77.2%           
=====================================
  Files        124     124           
  Lines      23021   23051   +30     
=====================================
+ Hits       17771   17800   +29     
- Misses      5250    5251    +1     

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