Function apply_grok_config 

pub fn apply_grok_config(
    path: &Path,
    stale: &str,
    bin: &str,
    args: &[String],
) -> Result<(ConfigEdit, Option<PathBuf>), String>

Applies rewrite_grok_config to a file: writes <file>.bak-<unix secs> first, then the new text through a temp file and rename. A missing file is created (nothing to back up).