Docs say WaterDrop "activates automatically with the Karafka framework" — but I can't reproduce that in 2.34.0? #6156

Hi! I might be missing something here, so wanted to check before assuming this is a bug.

docs/GettingStarted.md says:

The WaterDrop integration provides tracing of the waterdrop gem (a dependency of karafka, but also can be used standalone). This integration activates automatically with the Karafka framework.

I was debugging why Data Streams Monitoring wasn't showing a producer node for our Kafka topic, even with c.tracing.instrument :karafka enabled and DD_DATA_STREAMS_ENABLED=true set. Digging into lib/datadog/tracing/contrib/karafka/patcher.rb, the only WaterDrop-related thing it does is:

if Contrib::WaterDrop::Integration.compatible?
  ::Karafka.monitor.subscribe('app.initialized') do |event|
    ...
    Contrib::WaterDrop::Patcher.add_middleware(::Karafka.producer)
  end
end

That calls add_middleware directly, but never calls Contrib::WaterDrop::Patcher.patch — which is the method that does ::WaterDrop::Producer.prepend(Producer) and subscribes to 'producer.configured' (the thing that eventually attaches the message.acknowledged → track_kafka_produce listener DSM needs).

I wrote a small repro to check rather than just read the source:

require 'datadog'
require 'karafka'
require 'waterdrop'

Datadog.configure do |c|
  c.tracing.instrument :karafka, service_name: 'test-karafka'
  # no :waterdrop instrument call here
end

registry = Datadog::Tracing::Contrib::REGISTRY
puts registry[:waterdrop].patcher.patched?   # => false
puts registry[:karafka].patcher.patched?     # => true

On version 2.34.0, that prints false / true. Adding c.tracing.instrument :waterdrop, **options alongside :karafka flips it to true, and a freshly-created WaterDrop::Producer then actually has a message.acknowledged listener attached (confirmed via producer.monitor introspection).

So: is c.tracing.instrument :waterdrop supposed to be required separately even when using Karafka, and the docs just need updating? Or is there a different/newer activation path I'm not hitting (e.g. via Karafka::App.setup's own producer config, or some auto-instrument setting) that I'm missing in my repro? Happy to be wrong here — just didn't want to file a "docs are broken" issue without checking first.

Repro gem versions: datadog 2.34.0, karafka 2.5.8, waterdrop 2.8.16, Ruby 3.4.10.