Small improvements to PeriodicReader

[cijothomas]

Removed Mutex wrapping the channel, as channel.send/receive requires shared ref only.
Explicitly handle all outcome on try_recv.
Slightly improved internal logging.

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.3%. Comparing base (ce550e3) to head (0154773).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/periodic_reader.rs	73.3%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2421     +/-   ##
=======================================
- Coverage   79.3%   79.3%   -0.1%     
=======================================
  Files        122     122             
  Lines      21569   21565      -4     
=======================================
- Hits       17114   17110      -4     
  Misses      4455    4455             

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[lalitb]

not related - but should this be recv_timeout, to ensure that the main thread doesn't block indefinitely if the collect/export takes too much time? Similar for flush too.

[lalitb]

LGTM.