provide clear database usage stats in the WordPress admin area

[diosmosis]

Matomo can end up using up a lot of database space. This is expected since it's an analytics application, it tracks everything that happens, so for high traffic sites, that will equate to a lot of data. This may also not happen immediately but over time. It would thus be useful to keep the user notified about how much data is being used, especially since taking up space can cause issues with other aspects of WordPress (for example, a database backup plugin).

Information that would be useful to display to the user:

• percent of total database size used for matomo data
• amount of data in the log tables (including % of total database size)
• amount of data in the archive tables (including % of total database size)
• information regarding alternative products and migration paths for them so the user knows what their options are if they need to switch

If the percent of total database size is > a certain amount or the amount of data goes past a threshold we may want to consider showing a notification to the user. I'm unsure what the threshold should be however.