feat: add get_ref to retrieve Resource values by reference

[leghadjeu-christian]

Fixes #3163

Changes

Adds a new get_ref method to opentelemetry_sdk::Resource that returns a reference to the stored value instead of cloning it. This improves performance and avoids unnecessary allocations for read-only access, while keeping the existing get method for backward compatibility.

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: leghadjeu-christian / name: leghadjeu.christian (07fcace, 0a876c7, 48761ec, 971b870, b21e15b, bbf524d, bbf9ce7)
• ✅ login: scottgerring / name: Scott Gerring (48761ec)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.8%. Comparing base (3b2f751) to head (b21e15b).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3198   +/-   ##
=====================================
  Coverage   80.8%   80.8%           
=====================================
  Files        128     128           
  Lines      23090   23096    +6     
=====================================
+ Hits       18676   18682    +6     
  Misses      4414    4414           

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[scottgerring]

Hey @leghadjeu-christian thanks for raising this! Looks like a straightforward performance improvement with minimal side effects to me.

There's not really an established pattern of .get and .get_ref in the SDKs API, but I see no cleaner option to do this without a breaking change, which as you've pointed out we don't want.

The one other thing I note is that here:

opentelemetry-rust/stress/src/logs.rs

Line 71 in 3b2f751

.get(&Key::from_static_str("service.name"))

... could also be changed to use .as_ref !

[scottgerring]

Thanks - LGTM!