Add total projects imported usage ping

What does this MR do?

This MR adds projects_imported.total usage ping as per request in #224758 (closed) so that we can build a dashboard comparing total projects count vs the amount of projects imported.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content