Instrument usage_activity_by_stage_monthly.manage.user_auth_aggregate to support new Manage SMAU

Background

Currently, the Manage SMAU is the number of users viewing analytics features. This isn't the most accurate way to measure SMAU for the stage since it doesn't take into account all of the different personas Manage embodies.

As part of the new definition of Manage SMAU, we need a roll up metric that contains the aggregate, deduplicated users signing in via all providers.

Task

1. Verify the open question in this comment

2. Please create a new metric called:

usage_activity_by_stage_monthly.manage.user_auth_aggregate

This is the number of aggregate, deduplicated users signing in via all providers, as defined here:

- usage_activity_by_stage_monthly.manage.omniauth_providers #Number of unique user logins using an OmniAuth provider 
- usage_activity_by_stage_monthly.manage.user_auth_by_provider.two-factor # Number of unique user logins using two factor authentication
- usage_activity_by_stage_monthly.manage.user_auth_by_provider.two-factor-via-u2f-device #Number of unique user logins using two factor via a U2F device
- usage_activity_by_stage_monthly.manage.user_auth_by_provider.two-factor-via-webauthn-device #Number of unique user logins using two factor via a WebAuthn device
- usage_activity_by_stage_monthly.manage.user_auth_by_provider.standard #Number of unique user logins using password authentication
- usage_activity_by_stage_monthly.manage.user_auth_by_provider.google_oauth2 #Number of unique user logins using Google OAuth authentication