Fix some type issues for APM UI

[Ikuni17]

Summary

We're breaking #166813 up into smaller PRs in the interest of getting PRs through sooner for type fixes. These are the changes for APM UI.

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[elasticmachine]

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

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: f784de0

Failed CI Steps

• FTR Configs #68
• APM Cypress Tests #2

Test Failures

• [job] [logs] FTR Configs #68 / maps app embeddable maps add-to-dashboard save flow should allow existing maps be added by reference to an existing dashboard

Metrics [docs]

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
observabilityOnboarding	14	16	+2

Async chunks

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

id	before	after	diff
observabilityOnboarding	239.4KB	239.4KB	+10.0B

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
observabilityOnboarding	5.6KB	5.5KB	-57.0B

Unknown metric groups

API count

id	before	after	diff
observabilityOnboarding	14	17	+3

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @delanni @Ikuni17

[sorenlouv]

lgtm