[APM] transactions.kubernetes.pod can be undefined

[afharo]

Summary

We've been notified about some errors where the transaction view doesn't load because the transactions.kubernetes payload looks like this:

"kubernetes": {
  "namespace": "ci1-sass"
}

This PR makes kubernetes.pod an optional parameter in the Types definitions to make sure every logic validates it exists before reaching to its internal properties.

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately

Release Notes

Fixes issue that may show empty Transaction page in the APM app.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[afharo]

Q: Should the label point to 7.11.0?

[ogupte]

LGTM

[sorenlouv]

Thanks for fixing this @afharo ! I'm surprised this can even happen. We have a contract test between apm-server and kibana to ensure it doesn't.
Looking closer at our test I noticed that we don't test this properly. We test transactions here:

https://github.com/elastic/apm-integration-testing/blob/12b688db3b4f5166433532ccc53fbcad80147c77/scripts/kibana/validate-ts-interfaces-against-apm-server-sample-docs/scripts/download-sample-docs.ts#L23-L27

But we should test it against the minimal doc:
https://github.com/elastic/apm-server/blob/master/beater/test_approved_es_documents/TestPublishIntegrationMinimalEvents.approved.json

[sorenlouv]

Created this follow-up issue: #90065

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 768e041

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
apm	5.2MB	5.2MB	+118.0B

History

• 💚 Build #103078 succeeded 768e041

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