Monitoring of login access to reseller customers to CustomersDot

Problem

Add steps to monitor the access to reseller customers in production once we enable the feature flag. We need to make sure that customers are being given login access to CustomersDot correctly and they are shown the correct view (with subscription management buttons hidden and a banner stating why they can't make changes to subscriptions) once they login to CustomersDot.

Proposal

Resources we need to monitor during production release:

1. Sentry issues - https://new-sentry.gitlab.net/organizations/gitlab/issues/
2. Support channel - https://gitlab.slack.com/archives/C018C623KBJ

Actions we need to do during the release:

1. Find recently updated Reseller customers
   1. Run a script to detect which customers got access to CustomersDot; OR
   2. Run a Zuora report (reference).
2. Select a few customers for checks
3. Impersonate these users via https://customers.gitlab.com/admin/customer and check that UI elements are shown correctly (see #6560 (closed) for more info about it)
4. Check Mailgun to ensure that the necessary emails where delivered
5. Alternatively, check Salesforce for the Welcome emails.

See script

      activated_customers = Customer.includes(:billing_accounts).where(login_activated: true, billable: false)
      reseller_customers = []
      activated_customers.each do |customer|
        active_reseller_subscriptions = customer.subscriptions.select { |subscription| subscription.managed_by_reseller? == true && !subscription.expired?(with_grace_period: true) }
        reseller_customers.push(customer) if active_reseller_subscriptions.any?
      end

      print(reseller_customers.map { |customer| customer.email })

Note: Currently there is no product tracking (Snowplow) for this flow

Result

Next steps (if any)

How will we measure success?