fix(tracing): ensure sampling rate applies to whole trace

[samugi]

Summary

This commit fixes a bug where the sampling rate was applied to individual spans.

Problem: when sampling_rate is < 1 and the root span is not sampled, children (that are sampled) are reported without a parent so traces are split (producing multiple traces with different trace IDs for a single request).

The fix ensures that the sampled flag that is calculated by the sampler for the root span is propagated to all children in the same trace.

Checklist

• The Pull Request has tests
• There's an entry in the CHANGELOG
• (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-1926

[StarlightIbuki]

Basically OK.

[team-gateway-bot]

The backport to release/3.3.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/3.3.x release/3.3.x
# Navigate to the new working tree
cd .worktrees/backport-release/3.3.x
# Create a new branch
git switch --create backport-11135-to-release/3.3.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 cee8965e3e270e481d390f64a4960dcb5824ff63
# Push it to GitHub
git push --set-upstream origin backport-11135-to-release/3.3.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/3.3.x

Then, create a pull request where the base branch is release/3.3.x and the compare/head branch is backport-11135-to-release/3.3.x.