Fix Karafka framework patch

[Drowze]

What does this PR do?

Fixes the automatic WaterDrop integration activation (via the Karafka integration).

For context, by the time that Karafka::App#initialized! is called, Karafka.producer was already initialized.
Not only that, there's really nowhere we can hook in the Karafka initialization where we can be sure that Karafka.producer wasn't yet initiaized - that's because the Karafka initialization is NOT necessarily tied to the WaterDrop initialization. For instance, the following is a possible scenario:

$producer = WaterDrop::Producer.new { ... }

Datadog.configure { |c| c.tracing.instrument :karafka }

# note that the producer was initialized before the Karafka app
# (and in this case, even before datadog was configured)
Karafka::App.setup { |c| c.producer = $producer }

So instead of trying to hook somewhere before Karafka.producer is initialized, let's simply listen to a Karafka after-initialization event and append our middleware to the producer when that event is triggered.

Motivation:
We've noticed that WaterDrop distributed tracing was not working after configuring only the Karafka integration.

Change log entry

Additional Notes:

How to test the change?

[Drowze]

As of now, the WaterDrop integration requires version >= 2.8.8.rc1 (as it introduced producer.configured events, which we can hook onto to add a middleware to a just-initialized instance)
Meanwhile, the lowest Karafka version we support (2.3.0) requires waterdrop >= 2.6.12.

Because of that version discrepancy, it's perfectly possible to have Karafka compatible and WaterDrop incompatible at the same time (in fact, our karafka_min gemfiles fall into exactly this)