Fix existing Snowplow events adapted from Redis metrics to conform to a new standard - General (Gitlab.tracking.event)

Summary

This is a subset of the work described in &8641 (closed) per this comment, the work has been broken by emitting type.

General (Gitlab.tracking.event)

• usage_activity_by_stage_monthly.create.action_monthly_active_users_project_repo
• redis_hll_counters.user_packages.user_packages_total_unique_counts_monthly
• redis_hll_counters.terraform.p_terraform_state_api_unique_users_monthly
• usage_activity_by_stage_monthly.create.merge_requests_users

Testing

• Ensure related tests are updated