Report combined AMAU for Git write operations via usage ping
Problem to solve
The AMAU counters added by #219956 (closed) are separated by repository type (project, wiki, design), but the total distinct users is somewhere between the maximum and sum of these values. This means we are currently under or over counting Create SMAU for self hosted instances.
On GitLab.com this is not a problem because we can calculate it directly from the database dump.
Proposal
Add a new field that takes the distinct count of all users who performed any Git write operation to the usage_activity_by_stage_monthly section of the usage ping.