Allow aggregating CE events with EE events

What does this MR do and why?

Related to #369058 (closed)

We want to make it possible to use CE and EE events together in metrics.

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How to set up and validate locally

1. Create a new CE metric definition file using AggregatedMetric instrumentation_class and event names that combine both EE and CE events [by using event names that are defined in both CE and EE folder]
2. Open a rails console with FOSS_ONLY=1 env variable set and try creating a new service ping by running service_ping = ServicePing::BuildPayload.new.execute
3. To check the new metric's value in service ping, use the metric's key_path value like so:

key_path = xxx # use the 'key_path' value of the new metric here
service_ping.dig(dig(*key_path.split('.')))

This should return the value appropriate for the defined events and operation [AND/OR], skipping the listed EE events.

4. When repeating steps 2-3 in a rails console without FOSS_ONLY=1 set, the value returned in point 3 should not skip EE events.

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