You can do some or all of it in runtime.exs using environment variables. In your application’s start function, you can load config from files, the environment, etc. and use Application.put_env as @dimitarvp suggests.

defmodule My.Application do

  def start(_type, _args) do
    # set the config
    grafana_config = ....
    Application.put_env(:some_app, :key, grafana_config)

    # merge the config
    extra_some_config = ...
    existing_some_config = Application.get_env(:some_app, :some_config)
    Application.put_env(:some_app, :some_config, Keyword.merge(existing_some_config, extra_some_config)

    children = [...]
    # start the children
  end
end

To get it out of the repo, you’d have to start the Repo process first. You could try inserting a process in the children that starts after the repo but before promex and do the config in there.

Also check out Config Providers.