[REQUEST]: Make recommendation for `add_fields` as a means to support complex dynamic Kafka topics more prominent

[kpollich]

Description

We currently document the ability to use the add_fields processor to set a complex, interpolated dynamic topic value for outputting Elastic Agent data to Kafka here: https://www.elastic.co/docs/reference/fleet/kafka-output-settings#_topics_settings.

We should flesh this out with a specific example and a more explicit recommendation to use this behavior if the user requires a complex dynamic topic value.

Resources

https://www.elastic.co/docs/reference/fleet/kafka-output-settings#_topics_settings

Collaboration

The product team will provide the initial content.

Point of contact.

Main contact: @kpollich

Stakeholders: @elastic/fleet @elastic/elastic-agent-control-plane @strawgate @nimarezainia

[kpollich]

Processor example (needs review):

- add_fields: 
  target: ''
  fields: 
    kafka_topic: '${data_stream.type}-${data_stream.dataset}-${data_stream.namespace}

The proposed workflow here would be to add the block above to any integration policies on Fleet-managed agents or to the input config on any standalone agents that output to Kafka based on https://www.elastic.co/docs/reference/fleet/agent-processors.