Add 'observation-enabled' properties for RabbitMQ

[garyrussell]

Spring AMQP 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 BaseRabbitListenerContainerFactory and RabbitTemplate.

[mhalbritter]

Changes are here: https://github.com/mhalbritter/spring-boot/tree/mh/36451-add-observation-enabled-properties-for-rabbitmq

[sirlatrom]

Sorry if this is the wrong place to ask, but is this likely to be back ported to the 3.2.x or 3.1.x branches?