Docs on how to disable OpenTelemetry

[JoshMock]

• Adds unit tests to verify OTel disabling option
• Adds documentation for disabling OpenTelemetry

Fixes elastic/elastic-transport-js#284

[prodsecmachine]

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[github-actions]

🔍 Preview links for changed docs

• docs/reference/observability.md
• docs/reference/transport.md

[github-actions]

The backport to 8.19 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-8.19 8.19
# Navigate to the new working tree
cd .worktrees/backport-8.19
# Create a new branch
git switch --create backport-2934-to-8.19
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 feeaf155558a682ef2b5122940fb1a7f8bb607a0
# Push it to GitHub
git push --set-upstream origin backport-2934-to-8.19
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.19

Then, create a pull request where the base branch is 8.19 and the compare/head branch is backport-2934-to-8.19.