Add unique users using paid SAML features to usage ping (Access Paid GMAU)

Overview

We'd like to track Manage:Access usage across free and paid instances in #224102 (closed).

We'd also like to have similar instrumentation that only targets paid features to help us understand adoption.

Proposal

• Track unique users logging in with SAML SSO for Groups (https://docs.gitlab.com/ee/user/group/saml_sso/#saml-sso-for-gitlabcom-groups-premium) by adding a single aggregate count of distinct users to usage ping.