private long advanceNowAndComputeLatency() {
        final long previous = now;
        now = time.milliseconds();

        return Math.max(now - previous, 0);
    }
Compute the latency based on the current marked timestamp, and update the marked timestamp
with the current system timestamp.

@return latency
streams/src/main/java/org/apache/kafka/streams/processor/internals/StreamThread.java