[Logstash] [Stack Monitoring] Add new metric to check for pipelines missing a pipeline.id field

[justinkambic]

Summary

Resolves #24279.

As noted in the issue referenced above, older versions of Logstash (pre-6.4.0) will not display in Monitoring's Overview page unless a newer version of LS is also running. The reason this happens is also explained in comments on that issue.

The goal here is to add a second metric that will detect monitoring documents from older Logstash instances. Essentially, if there are any instances without a pipeline.id field, we would treat that as a case where we should ignore the special logic that runs to hide LS until the monitoring index has accrued a sufficient amount of data.

Checklist

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• This was checked for cross-browser compatibility, including a check against IE11
• 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
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[justinkambic]

Closed in favor of #34487.