Function subscriber 

pub fn subscriber<W>(
    json: bool,
    filter: EnvFilter,
    writer: W,
    traces: Option<&SdkTracerProvider>,
) -> Box<dyn Subscriber + Send + Sync>
where
    W: for<'w> MakeWriter<'w> + Send + Sync + 'static,

The subscriber init_logging installs, writing to writer: JSON lines when json, otherwise text for people; and with traces, sending each span on as a trace too.