Add MAU data for Ecosystem Usage Ping

Data needed

Currently, we only collect the number of active services per integration type, but we'd also like to know the number of users of each integration.

I'd like to add another set of data along the lines of projects_FOO_users. This should be a unique count of the number of members of each project that has an active service.

Links/resources

• def services_usage in usage_data.rb - https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/usage_data.rb#L348

/cc @leipert