Add usage ping data for File locking feature

Description

As we are moving away from the notion of Products (https://gitlab.com/gitlab-org/gitlab-ee/issues/1641#note_26302222), new licenses won't have neither Geo or File locking as Products in the license key.

That means we won't be able to simply count the number of customers who have the File locking addon to know how many customers use File Locking.

Proposal

• We need to add a counter, locked_files, on the usage ping to know how many files are locked at the time the usage ping is sent.
• projects_with_locked_files: number of projects that have any files locked.