Add observationEnabled properties for Apache Kafka

[garyrussell]

Spring for Apache Kafka does not enable Observation by default; if a user only wants Micrometer timers, it is better to use the legacy timers provided by the project, which detects the presence (or not) of a MeterRegistry bean.

However, if a user wants tracing, the observationEnabled property must be set to true; it would be more convenient if Boot provided this as an auto config property.

Furthermore, if Boot can detect that tracing is enabled (not just timers via Observation), then the properties could default to true, if not set by the user. If no such detection is possible, then the properties should remain at their default, false.

The observationEnabled property is available on ContainerProperties and KafkaTemplate.

[tspoeri]

Very handy enhancement that I am also missing. It is quite annoying to write a manual configuration just because of that property when the rest can be configured through auto configuration.

[scottfrederick]

Closing in favor of #38057