Monitor number of dropped elements due to lateness in Samza runner

[Sören Henning]

Hey community,

I'm interested in how we can monitor the number of elements dropped that 
arrive too late for a windowed aggregation (i.e., allowed lateness 
passed). I configured a JMX reporter for Samza (and export these JMX 
metrics to Prometheus). However, while I get a number of both Samza and 
Beam metrics, I neither find a Samza nor a Beam metric reporting on the 
number of dropped/skipped messages.

In the Beam sources, I found that a "droppedDueToLateness" metric 
(https://github.com/apache/beam/blob/master/runners/core-java/src/main/java/org/apache/beam/runners/core/LateDataDroppingDoFnRunner.java) 
exists and the Samza documentation reports on "window-skipped" 
(https://samza.apache.org/learn/documentation/latest/container/metrics.html). 
However, both metrics are not reported for my pipeline and I don't find 
any further explanation for them.

Thanks
Sören