Improve shutdown handling in PeriodicReader

[cijothomas]

If shutdown is called from multiple threads, modify the logic so that only one enters the core logic of sending shutdown signal etc. Previously, both threads can send shutdown message to background thread, and after the 1st one, the thread exists, so the other shutdown fails. Though it is okay, the error message may not be super easy to follow.
This PR makes sure only one shutdown proceeds, the other one immediately returning a failure indicating that failure is due to shutdown already initiated. This also ensures, checking for shutdown flag is only done at user thread invoking shutdown/flush.

There is a similar race between Flush and Shutdown possible, added it as a TODO to improve log messaging.

Also added more test for drop.

[codecov]

Codecov Report

Attention: Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.3%. Comparing base (9b0ccce) to head (21bbd6b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/periodic_reader.rs	92.1%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2422   +/-   ##
=====================================
  Coverage   79.3%   79.3%           
=====================================
  Files        122     122           
  Lines      21565   21588   +23     
=====================================
+ Hits       17110   17134   +24     
+ Misses      4455    4454    -1     

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[utpilla]

Left some suggestions.