Move projects_imported_from_github from Service ping metric outside ee
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=336871)
</details>
<!--IssueSummary end-->
Looking at metric [implementation](https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/usage_data.rb#L252) this is the count of creator ids from projects imported from GitHub
Checking [docs](https://docs.gitlab.com/ee/user/project/import/github.html) 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`
issue