Add unique users importing groups to 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 to usage ping.
• This number will count all attempted imports (not successful completion).
• Since the number of distinct importers (actions) is large, this feature will be implemented in multiple issues
• Following (group) importers are in scope for this issue:
  • GitLab Group importer (API and UI)