PeriodicReader shutdown modified to enforce timeout

[cijothomas]

Just hardcoded timeout now. Will make it configurable in next.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 77.9%. Comparing base (5aa9120) to head (02478fd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/periodic_reader.rs	50.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2513     +/-   ##
=======================================
- Coverage   77.9%   77.9%   -0.1%     
=======================================
  Files        123     123             
  Lines      22949   22953      +4     
=======================================
+ Hits       17880   17881      +1     
- Misses      5069    5072      +3     

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