@spec metadata(state :: map(), adaptor_path :: String.t()) ::
  {:ok, Lightning.CLI.Result.t()} | {:error, :enoent | :timeout}

Retrieve metadata for a given adaptor and configuration.

The state is handed to the CLI in a file inside a directory only this user can enter, rather than on the command line, where other processes could read it. The directory is private before the file exists, so there is no window where the file is readable by others.