[Infra Monitoring UI] Create functional test for Stack Monitoring Agent collection using generated data

[miltonhultgren]

Overview

We want to move further with tooling for data generation to use for testing and development. After some initial work done in #118302 we now want to go further with apm-synthtrace and integrate it with our functional tests.

Since we need to write a new set of tests for testing Agent based Stack Monitoring we felt this would be a good opportunity to extend our data generation to simulate the data that Agent based collection would produce.

Note that if this ends up taking more time than we first hoped, it may be best to pause this effort, and instead write the tests by using es_archive.

AC

There exists at least one functional test for Stack Monitoring that uses generated data that matches the format that Agent based collection uses.

Implementation hints

Start by looking at the initial PR, that shows how we use synthtrace in general (to generate timestamps, and push those through the document generating classes).
Then looking at those classes you can see what kind of documents they output.

Next we'd need to compare what the archive versions include since that is what we need to start generating.

Then it's all about deciding which domain class should provide the data for those fields.

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)