Add data_source `internal_events` for new API

What does this MR do and why?

This MR adds a new data_source for metrics for use with internal events tracking, which combines firing both Redis HLL and Snowplow events associated with the metric at the same time.

I added a new source for the internal_events in the logic for aggregation metrics, but it's the same as Redis HLL, since we haven't implemented support for ordinary Redis metrics yet, so the behavior should be identical.

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Related to #411454 (closed)