Fix metric collections during PeriodicReader shutdown

[lalitb]

A nit fix in metric collection during PeriodicReader shutdown.

The PeriodicReader performs a one last collection before shutting down. However, currently, the shutdown flag is set before the message is sent to initiate the collection process asynchronously. This result in situation where collection error's out as the reader is already shutdown:

     Finished dev [unoptimized + debuginfo] target(s) in 6.60s
     Running `/<path-to>/opentelemetry-rust/target/debug/metrics-basic`
     Error: Other("[Other(\"reader is shut down\")]")

The proposed PR resolves this by ensuring that the shutdown flag is only set after the completion of the collection."

Changes

Please provide a brief description of the changes here.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (c06c04b) 55.4% compared to head (9c0cfd0) 55.4%.

Files	Patch %	Lines
opentelemetry-sdk/src/metrics/periodic_reader.rs	0.0%	6 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1375   +/-   ##
=====================================
  Coverage   55.4%   55.4%           
=====================================
  Files        147     147           
  Lines      18083   18084    +1     
=====================================
+ Hits       10021   10022    +1     
  Misses      8062    8062           

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[jtescher]

Good call