fix: KeyValueMetadata to not expose its fields public

[cijothomas]

Don't think public was required, and this should be transparent to users anyway...
https://github.com/open-telemetry/opentelemetry-rust/pull/2837/files#r2008002156

[Copilot]

Pull Request Overview

This PR changes the visibility of the fields in KeyValueMetadata from public to crate-level, ensuring that internal implementation details remain encapsulated and transparent to users.

• Changed KeyValueMetadata field visibility in opentelemetry/src/baggage.rs
• Updated the CHANGELOG.md to document the new encapsulation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
opentelemetry/src/baggage.rs	Changed field visibility from pub to pub(crate) for encapsulation
opentelemetry/CHANGELOG.md	Updated changelog entry to reflect the new visibility of KeyValueMetadata

[gruebel]

👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.9%. Comparing base (3e77a22) to head (ce94863).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2843   +/-   ##
=====================================
  Coverage   80.9%   80.9%           
=====================================
  Files        124     124           
  Lines      23726   23726           
=====================================
  Hits       19200   19200           
  Misses      4526    4526           

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