Baggage improvements

[fraillt]

Changes

Improve few things:

• BaggageExt::with_baggage now "clone" baggage, instead of iterate+collect.
• Baggage::insert_with_metadata (used everywhere) should be a bit faster, as it had redundant calculations:
  • access hashmap once (before it was twice)
  • calculate entry content length once (before it was three times, although it's so trivial that probably it's not noticeable anyway)
• updated incorrect comment on Baggage::iter

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)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: fraillt / name: Mindaugas Vinkelis (8438828, 02cac17)

[codecov]

Codecov Report

Attention: Patch coverage is 68.42105% with 12 lines in your changes missing coverage. Please review.

Project coverage is 79.6%. Comparing base (45ed2e0) to head (a20d014).

Files with missing lines	Patch %	Lines
opentelemetry/src/baggage.rs	68.4%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2284     +/-   ##
=======================================
- Coverage   79.6%   79.6%   -0.1%     
=======================================
  Files        123     123             
  Lines      21263   21255      -8     
=======================================
- Hits       16938   16926     -12     
- Misses      4325    4329      +4     

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