Function synthesize_probes 

fn synthesize_probes<'scope, T: Timestamp>(
    source_id: GlobalId,
    progress: StreamVec<'scope, T, Infallible>,
    interval: Duration,
    now_fn: NowFn,
) -> StreamVec<'scope, T, Probe<T>>

Synthesizes a probe stream that produces the frontier of the given progress stream at the given interval.

This is used as a fallback for sources that don’t support probing the frontier of the upstream system.