Add "Contributor count" metric to Value Streams Dashboard

What does this MR do and why?

Adds the Clickhouse-based "Contributor count" metric to the comparison panel in the Value Streams Dashboard. It is only available at the group level and when Clickhouse has been enabled.

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View	Before	After
Built-in VSD
Dynamic VSD

How to set up and validate locally

Enable these feature flags:

# enable Clickhouse
Feature.enable(:clickhouse_data_collection)
Feature.enable(:event_sync_worker_for_click_house)

# enable group dashboards and dynamic group VSD
Feature.enable(:group_analytics_dashboards)
Feature.enable(:group_analytics_dashboard_dynamic_vsd)

1. Please make sure that you have a GitLab Ultimate license.
2. Ensure that Clickhouse is configured: https://docs.gitlab.com/ee/development/database/clickhouse/clickhouse_within_gitlab.html
   • For prepping the DB schema you can invoke: bundle exec rake gitlab:clickhouse:migrate
3. FILTER=productivity_analytics SEED_PRODUCTIVITY_ANALYTICS=1 bundle exec rake db:seed_fu
4. Visit the group from the output above, get its ID and run the following seeder: GROUP_ID={GROUP_ID} SEED_VSD_COUNTS=1 FILTER=vsd_overview_counts bundle exec rake db:seed_fu
5. In your rails console, enter the following: ClickHouse::EventsSyncWorker.new.perform
6. Visit your group's VSD and verify that the new metric appears in the comparison panel: http://gdk.test:3443/groups/{GROUP_PATH}/-/analytics/dashboards/value_streams_dashboard
   • To verify in the built-in VSD as well, disable the group_analytics_dashboard_dynamic_vsd feature flag and refresh the page

Related to #433353 (closed)