Explore options for Gitaly Custom Config with 16.0+ format
Follow up from !986 (merged).
With Gitaly and Praefect config format changing in 16.0+ to be nested under a top level ['configuration']
key this introduces a small challenge here for Custom Config.
We utilise the ability for Omnibus to import additional config via it's from_file
option. However if a user now looks to specify a specific setting under the top key this will overwrite all other options as Omnibus merges at the top key and not inline.
Task is to explore options to see if we can make this more graceful for users.