Review: [Stack Monitoring] Logstash migration

[matschaffer]

For elastic#113256

Summary

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Checklist

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• 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

Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to identify risks that should be tested prior to the change/feature release.

When forming the risk matrix, consider some of the following examples and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes | ---------------------------|-------------|----------|-------------------------|
| Multiple Spaces—unexpected behavior in non-default Kibana Space. | Low | High | Integration tests will verify that all features are still
| supported in non-default Kibana Space and when user switches between spaces. | Multiple nodes—Elasticsearch polling might have race conditions
| when multiple Kibana nodes are polling for the same tasks. | High | Low | Tasks are idempotent, so executing them multiple times will not result in
| logical error, but will degrade performance. To test for this case we add plenty of unit tests around this logic and document manual testing
| procedure. | Code should gracefully handle cases when feature X or plugin Y are disabled. | Medium | High | Unit tests will verify that any feature
| flag or plugin combination still results in our service operational. | See more potential risk
| examples |

For maintainers

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

------------------------ >8 ------------------------ Do not modify or remove the line above. Everything below it will be ignored.

Requesting a pull to phillipb:logstash-migration from matschaffer:logstash-migration

Write a message for this pull request. The first block of text is the title and the rest is the description.

[matschaffer]

Closing in favor of elastic#114218