feat: add an Iterator for opentelemetry::trace::TraceState key-value pairs

[iunanua]

Changes

Implement IntoIterator for TraceState by relying into the TraceStateIter iterator.

Notes

Now the only way to extract the different entries of the TraceState is using the header() or header_delimited() methods, making it impossible to avoid allocations.
That's why providing an iterator can be valuable in certain cases where it's necessary to know the TraceState entries while avoiding serialization.

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 80.9%. Comparing base (d721037) to head (997ddc9).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3164   +/-   ##
=====================================
  Coverage   80.8%   80.9%           
=====================================
  Files        128     128           
  Lines      23090   23136   +46     
=====================================
+ Hits       18676   18722   +46     
  Misses      4414    4414           

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[scottgerring]

Hey @iunanua thanks for the contribution, and sorry for the slow turnaround!

This LGTM and provides a clear perf improvement when needed; I also note there is no other way to do this from outside the crate, and it extends a similar pattern already present for Baggage and the Resource itself:

opentelemetry-rust/opentelemetry-sdk/src/resource/mod.rs

Lines 245 to 252 in 3b2f751

	impl<'a> IntoIterator for &'a Resource {
	type Item = (&'a Key, &'a Value);
	type IntoIter = Iter<'a>;
	fn into_iter(self) -> Self::IntoIter {
	Iter(self.inner.attrs.iter())
	}
	}

opentelemetry-rust/opentelemetry/src/baggage.rs

Lines 254 to 261 in 3b2f751

	impl<'a> IntoIterator for &'a Baggage {
	type Item = (&'a Key, &'a (StringValue, BaggageMetadata));
	type IntoIter = Iter<'a>;
	fn into_iter(self) -> Self::IntoIter {
	Iter(self.inner.iter())
	}
	}