add KafkaProducer post-processor to enable extensions from other libraries

[sszp]

Context

I'd like to enable async traces for messages sent out with tw-tkms.

To able to use OpenTelemetry's KafkaTelemetry class from the tw-observability-base, I'm adding a new KafkaProducer post-processor interface that can be used from tw-observability-base to call the KafkaTelemetry:wrap() to create the necessary async traces for all message sent out with tw-tkms.

Checklist

• Change meets or does not compromise the Baseline Security Requirements

[tw-peeterkarolin]

I think it would make sense to play this through with a snapshot version in combination with tw-observability-base to understand how the whole flow will work.
Just using KafkaTelemetry:wrap() won't be enough, because that that will work with normal use cases where the OpenTelemetry context is configured and available for the KafkaTelemetry wrapper to use. In tw-tkms polling the messages from the DB and publishing them to Kafka will happen in a separate poller thread, so it will need to initialise the OpenTelemetry context as well, before KafkaTelemetry can work.

[sszp]

I think it would make sense to play this through with a snapshot version in combination with tw-observability-base to understand how the whole flow will work. Just using KafkaTelemetry:wrap() won't be enough, because that that will work with normal use cases where the OpenTelemetry context is configured and available for the KafkaTelemetry wrapper to use. In tw-tkms polling the messages from the DB and publishing them to Kafka will happen in a separate poller thread, so it will need to initialise the OpenTelemetry context as well, before KafkaTelemetry can work.

I plan to look into that. At first I plan to see if I can use ITkmsMessageDecorator to add headers to the message and get those headers from the message using ITkmsMessageInterceptors just before it's sent out using the Kafka Client to initialise the OpenTelemetry context before sending.

[normanma-tw]

LGTM once the snapshot version is cleaned up + optionally update the date of the changelog

[tw-peeterkarolin]

Probably would be better to declare the autowire usage as
@Autowired(required = false)
Then you don't need to explicitly provide an empty list bean.

[sszp]

Fixed that.

[sszp]

LGTM once the snapshot version is cleaned up + optionally update the date of the changelog
That's done now. @normanma-tw