public void markIn() {
        if (context.get() != null) {
            throw new IllegalStateException("MarkIn consecutively called without calling markOut in " + metricName);
        }
        context.set(execLatencyTimer.get().time());
    }
This is called when the processing of the event is started. This is
called at ProcessStreamReceiver#receive before the event is passed into
process chain
modules/siddhi-core/src/main/java/io/siddhi/core/util/statistics/metrics/SiddhiLatencyMetric.java