Calculate xMAUs unions for Redis HLL metrics

What does this MR do?

This MR demonstrates proposed implementation for new feature (https://gitlab.com/gitlab-org/product-analytics/-/issues/421) that will allow for calculation unions over multiple tracked metrics in order to get more accurate data for GMAUs and SMAUs metrics aggregation.

Baseline idea is as follow:

1. PM defines events combination in new YAML file lib/gitlab/usage_data_counters/aggregated_metrics.yml
2. As the final step of usage ping generation, when all events are already calculated new method Gitlab::UsageDataCounters::HLLRedisCounter.combined_events_data is called
3. .aggregated_metrics_data reads content of lib/gitlab/usage_data_counters/aggregated_metrics.yml and calculates number of unique elements in union of all listed metrics for given events combination, with usage of pfcount

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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