Group migration - Instrument usage ping (Import GMAU)

Problem

In gitlab-org/manage/general-discussion#17232, we described the need for a Group MAU for Import. This should be a measure of the number of unique users taking an action that is important to the goals of the group.

Proposal

• Add the number of unique users importing groups using the new API-based group import to usage ping.
• This number will count all attempted imports (not successful completion).
• This count should be distinct from the current Group imports (using file-based group exports), so that we can tell them apart.