Add aggregate metrics event name validation

What does this MR do and why?

Related to #369058 (closed)

As a first step towards EE-CE combined metrics, we want to introduce validation to make sure undefined event names cannot be used in metrics definitions. We want this validation to happen only on EE, so that EE-only events would not cause errors when used in CE metric definitions.

Screenshots or screen recordings

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How to set up and validate locally

1. Create a new metric definition file using AggregatedMetric instrumentation_class and undefined event names [event names that can't be found in the known_events yml files]
2. Run the Every metric definition spec file
3. This test should fail, raising an error informing about using undefined event names.

MR acceptance checklist

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