new stat: how long it took to land an update from internal dist-git to source-git

[TomasTomecek]

Suggested by Stef.

Since we'll be running this service internally, we'll have access to all the data.

Stef suggested to create a stat how long it takes to get an internal build to CentOS Stream. Sounds interesting, though implementation could be pretty rough - how to get the date when the internal change happened - what would we actually measure? commit date, %changelog date, build date, "added to an advisory" date