Function layer 

pub fn layer(
    config: FileLoggerConfig,
) -> Result<(LogCollector, impl Layer<Registry> + Send + Sync + 'static, WorkerGuard), InstallError>

Builds the cloud-controlled file layer and its LogCollector without installing a global subscriber. Use this when composing the file layer with other layers; the returned layer is pre-wrapped in the reloadable filter so the cloud’s SetLogLevel overrides take effect.