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 (e5d29d9, ef45ca7)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.6%. Comparing base (c75413c) to head (ef45ca7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/resource/mod.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3198     +/-   ##
=======================================
- Coverage   80.7%   80.6%   -0.1%     
=======================================
  Files        128     128             
  Lines      23195   23198      +3     
=======================================
  Hits       18719   18719             
- Misses      4476    4479      +3     

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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!

[cijothomas]

I have requested for some additional information on the need of this new method here.

Irrespective of that, I am fine adding the new method. One comment - unless really needed, let's keep all existing usage unmodified in this PR. If needed, I'd prefer to have a follow up PR updating all usages to the new method. This helps keep PRs short and easy to see that this is purely additive change.

[cijothomas]

The change is good. I suggest to not modify test code. etc to use the new method in this PR.