Unify 'observation-enabled' property defaults if possible

[philwebb]

See #36451 and #38644 for examples.

Currently some technologies have an observation-enabled property which can be used to enable/disable observations. We should try to align the default value of this property for all of them.

There may be performance reasons why we currently don't enable all observations by default.

[mhalbritter]

Currently, we have:

• spring.kafka.listener.observation-enabled with default false
• spring.kafka.template.observation-enabled with default false
• spring.pulsar.listener.observation-enabled with default true

After #36451, we additionally have:

• spring.rabbitmq.template.observation-enabled with default false
• spring.rabbitmq.listener.simple.observation-enabled with default false
• spring.rabbitmq.listener.direct.observation-enabled with default false
• spring.rabbitmq.listener.stream.observation-enabled with default false

[mhalbritter]

Superseded by #39276.