fix(tracing): set parent span correctly

[samugi]

Note: the bug was introduced by #11468 where the parent_id was incorrectly updated on the balancer span instead of the root span. This PR reverts that part.

Summary

When the balancer instrumentation was enabled, the parent span was set incorrectly on traces, this fix addresses the problem by setting the parent span correctly on the root (kong) span when there is an incoming tracing header.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• (no) There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• [Implement ...]

Issue reference

KAG-2811
FTI-5485

[nowNick]

Looks good to me!
🚀

[team-gateway-bot]

The backport to release/3.4.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release/3.4.x release/3.4.x
# Navigate to the new working tree
cd .worktrees/backport-release/3.4.x
# Create a new branch
git switch --create backport-11786-to-release/3.4.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 72580d5ff18fcc5cdf994a097dae8eb16215ff92
# Push it to GitHub
git push --set-upstream origin backport-11786-to-release/3.4.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/3.4.x

Then, create a pull request where the base branch is release/3.4.x and the compare/head branch is backport-11786-to-release/3.4.x.

[team-gateway-bot]

The backport to release/3.5.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release/3.5.x release/3.5.x
# Navigate to the new working tree
cd .worktrees/backport-release/3.5.x
# Create a new branch
git switch --create backport-11786-to-release/3.5.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 72580d5ff18fcc5cdf994a097dae8eb16215ff92
# Push it to GitHub
git push --set-upstream origin backport-11786-to-release/3.5.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/3.5.x

Then, create a pull request where the base branch is release/3.5.x and the compare/head branch is backport-11786-to-release/3.5.x.