NMS-13176: add testing module including a benchmark app

[swachter]

Issue: https://issues.opennms.org/browse/NMS-13176

• Adds the testing module that contains functionality to generate synthetic flows, write those flows to Kafka, or use them directly in a pipeline. In addition, a benchmark app can measure the throughput and processing time of pipeline runs.
• Some minor changes to the pipeline to improve testability. E.g. the TimestampPolicyFactory must now be supplied when constructing a ReadFromKafka instance. This was necessary because the benchmark app needs a slightly patched timestamp policy factory.
• A minor change how windowing parameters are passed around
• Replaces distribution metrics for Kafka and ES drift by gauges. (Distribution metrics do not work with in the combination Beam/Flink/Prometheus.)

[fooker]

Some minor nits and some proposals for the cardinality of some dimensions of the generated flows. I'm not sure if this is relevant for the current test but seems nice to have these available.

[fooker]

Maybe use resilience4j RateLimiter instead?

[swachter]

I would keep the current implementation. Its minimal and does exactly what is needed.

[fooker]

I would turn this around: Is there any reason not to use an implementation somebody else takes care for?

[swachter]

resilience4j.RateLimiter seems to be an overkill here. RateLimiterConfig, RateLimiterRegistry, and RateLimiter are lots of ceremony. More severly: Looking through the documentation I did not see if there is a check(numberOfPermits) method.

[fooker]

There is reservePermission(numberOfPermits). If it returns 0 the call is permitted.
The registry is completely optional and the config is a temporary object.

[swachter]

I spent some time on resilience4j.RateLimiter and still think it is way too complicated. There seems to be no simple possibility to disable it. In addition, our limiter has not to be thread-safe. The custom Limiter implementation is simple. There is no maintenance burden. I see no benefit to use a dependency as a replacement.

[fooker]

¯\(ツ)/¯