Add # of unique users creating groups to usage ping

Problem

In https://gitlab.com/gitlab-org/telemetry/issues/293, we provided a more specific need to define AMAU for stages. You can find the definition for AMAU and SMAU here.

Manage has no appropriate AMAU metrics in usage ping; the KPIs we currently capture are cumulative counts that don't count toward an active user taking a specific action. As a first step, we should use the number of users creating groups as a Manage AMAU.

Please see gitlab-org/growth/product#1484 for more context.

Proposal

Add the unique number of users creating groups to usage ping.

See an example for Create.

Update:

Based on #209839 (comment 302853273), to start with, we report the number of users created.