[hidingmyname]

Part of #4745

Changes

Baggage::ToHeader encoded a space in keys/values as + (form-style). The W3C Baggage specification uses percent-encoding, and other OpenTelemetry implementations read + as a literal plus sign, so a value such as hello world arrived as hello+world on non-C++ receivers.

This PR changes only the encode side: a space is now emitted as %20 (it simply falls through to the existing percent-encoding branch). As noted in the issue triage, this is safe across versions because existing C++ readers already decode %20.

The decode side is intentionally unchanged: FromHeader still decodes + as a space, so headers from older C++ senders keep reading the same way. Whether to stop treating + as a space is left as a maintainer decision; I'm happy to follow up with a separate PR for that if wanted.

Tests (api/test/baggage/baggage_test.cc):

• ValidateInjectHeader: expected headers updated from + to %20.
• ValidateExtractHeader: still asserts + decodes to a space; added a case that %20 decodes to a space.
• New SpaceIsPercentEncoded: Set("k", "hello world")->ToHeader() is k=hello%20world and round-trips through FromHeader.

Verification (GCC 15.3, CMake + Ninja, Debug):

• Without the header change, the updated tests fail: baggage_test 12 passed, 2 failed (ValidateInjectHeader, SpaceIsPercentEncoded, which got k=hello+world).
• With the change: baggage_test 14/14, baggage_propagator_test 6/6, composite_propagator_test 2/2, global_propagator_test 3/3, propagator_builders_test 11/11 passed.
• clang-format 18 applied.

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.61%. Comparing base (d05c6a8) to head (a93090a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4765      +/-   ##
==========================================
- Coverage   86.61%   86.61%   -0.00%     
==========================================
  Files         530      530              
  Lines       21169    21167       -2     
==========================================
- Hits        18333    18331       -2     
  Misses       2836     2836              

Files with missing lines	Coverage Δ
api/include/opentelemetry/baggage/baggage.h	98.75% <ø> (-0.01%)	⬇️

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[mateenali66]

LGTM, thanks for the fix.

ValidateInjectHeader and SpaceIsPercentEncoded fail with the + branch put back and pass at 7aa97087, and + still decodes as a space.

See minor comment on the CHANGELOG link.