Add an example demonstrating the lock contention for metrics

[martintmk]

When looking at #1386 I wanted to practically demonstrate the lock contention for metrics and how it affects the performance.

To run the example using a shared MeterProvider:

cargo run --release -- shared

To run the example using a per-thread MeterProvider:

cargo run --release -- per-thread

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

All modified and coverable lines are covered by tests ✅

Project coverage is 79.0%. Comparing base (eeb367d) to head (3e1c1bc).
Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2651     +/-   ##
=======================================
- Coverage   79.1%   79.0%   -0.2%     
=======================================
  Files        119     120      +1     
  Lines      22556   22512     -44     
=======================================
- Hits       17857   17791     -66     
- Misses      4699    4721     +22     

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[cijothomas]

Thanks! Please leverage the existing stress test https://github.com/open-telemetry/opentelemetry-rust/tree/main/stress/src and add a new scenario which use per-thread provider.

[martintmk]

Replaced by #2659