How to monitor the latency?

[FatMouse]

Hello:


I hava set the LatencyTrackingInterval as 2000,but int the `Task Metrics` 
latency was always Nan.How can I monitor the latency?Thanks.





Code:


StreamExecutionEnvironment env = 
StreamExecutionEnvironment.getExecutionEnvironment();
env.getConfig().setLatencyTrackingInterval(2000);



Best,


MAtrix42