Spike: Add monitoring for SaaS Update Plan functionality in a CDot Customers account

Problem

As the current update plan functionality can be used to alter the subscription of a customer, we should be able to monitor this properly.

The functionality is available for all CDot admins that have customers read_write access. The functionality to update plan is accessible if a CDot admin navigates to:

1. Customers
2. Pick the dedicated customer account
3. GitLab Groups
4. Drop down beneath Plan and corresponding namespace
5. Once switched and selected, clicks Update

Example on Staging: https://customers.staging.gitlab.com/admin/customer/237623/list_gitlab_namespaces

The functionality gets mostly used by Support and is also available via a mechanizer function. The corresponding handbook page can be found here.

Proposal

We should add the update plan functionality to papertrail in the corresponding customer account, so we can see in the History tab who, when and what was updated regarding the plan.

Currently the papertrail for Update plan is only available in the corresponding order if a plan gets updated by that functionality.

Further, we can think about introducing a new field that a reason needs to be provided when this update happens and add it to the papertrail.

Lastly, we could also think about introducing a statistics panel for this functionality.

Result

We are able to monitor the Update plan functionality inside CDot admin and can do deeper anaylsis on the interactions if needed.