Introduce new New Service Ping metric - UMAU

What does this MR do and why?

Closes https://gitlab.com/gitlab-org/product-intelligence/-/issues/549

the business would like to create a new UMAU metric.

The metric is defined by:

1. These are users that have a last_activity_on date within the past 28 days.
2. Noting that last_activity_on encompasses a broader set of actions and is a more inclusive measure of active users (picks up active users even when they are not using the UI).
3. This metric indicates any level of activity.

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