fix: add missing span kind to otel traces

[kruskall]

Motivation/summary

ensure self instrumentation traces have a span kind

Checklist

• [ ] Update CHANGELOG.asciidoc
• [ ] Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

Closes https://github.com/elastic/apm-server/issues/19441

[github-actions[bot]]

:robot: GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify[bot]]

This pull request does not have a backport label. Could you fix it @kruskall? 🙏 To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

:green_heart: Build Succeeded

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• Commit: 59e9e951b5b2f8d471dd9db83b9fbf16e6b5679c