PeriodicReader - alternate shutdown implementation

[cijothomas]

An alternate way of handling shutdown - this just sends a message to the background thread to initiate shutdown, and wait for thread handle to join.
The handle.join() has some risks - as shutdown could get block indefinitely if the background thread is stuck in a export, and we cannot enforce timeout :(

An alternative way would be keep the existing way, but enforce recv_timeout, so that shutdown always ends in a fixed time, no matter whether background is stuck or not. Will send a PR with this approach as well, but either way, if a rogue exporter is used, background thread is still stuck. All we can ensure is shutdown gets a timed exit. (esp. important since shutdown can be triggered automatically when Drop() occurs...)

Not for merge, but to have a discussion about various approaches.

[codecov]

Codecov Report

Attention: Patch coverage is 60.71429% with 11 lines in your changes missing coverage. Please review.

Project coverage is 77.8%. Comparing base (8fde6eb) to head (7e6f2e4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/periodic_reader.rs	60.7%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2482     +/-   ##
=======================================
+ Coverage   77.2%   77.8%   +0.5%     
=======================================
  Files        124     123      -1     
  Lines      23043   22875    -168     
=======================================
+ Hits       17800   17805      +5     
+ Misses      5243    5070    -173     

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