Move projects_imported_from_github from Service ping metric outside ee

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Looking at metric implementation this is the count of creator ids from projects imported from GitHub

Checking docs I see this is available in all tiers.

Consider moving the metric implementation and metrics definitions outside ee directory

Service ping implementation

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/usage_data.rb#L252

Key path:

• key_path: usage_activity_by_stage_monthly.create.projects_imported_from_github
• key_path: usage_activity_by_stage.create.projects_imported_from_github

Update product_category: group::source code