Remove global shutdown from metrics and doc fixes

[cijothomas]

Fixes https://github.com/open-telemetry/opentelemetry-rust/issues/1679

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.6%. Comparing base (348ec9e) to head (fe4dd4c). Report is 19 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1743     +/-   ##
=======================================
+ Coverage   71.0%   71.6%   +0.5%     
=======================================
  Files        135     136      +1     
  Lines      20751   20829     +78     
=======================================
+ Hits       14746   14924    +178     
+ Misses      6005    5905    -100     

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[lalitb]

Just to summarise my understanding of the existing behavior after this PR:

• The global shutdown is now removed for metrics and logs. And shutdown will happen when
  • explicitly invoked from any of the Sdk*Provider's, Or
  • All the references to the providers are dropped.
• From traces, global shutdown will drop the global provider, and invoke shutdown (through drop) if there are no other provider references.