Migrate users_viewing_analytics_group_devops to Snowplow

What does this MR do and why?

Related to #362274 (closed)

This MR migrate first metric (analytics_unique_visits.analytics_unique_visits_for_any_target_monthly) to Snowplow.

New Feature flag route_hll_to_snowplow_phase2.

Old code related to route_hll_to_snowplow also refactored to accomodate new phase.

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