The `JmsMessageDrivenEndpoint` delegates all the hard work to the
`ChannelPublishingJmsMessageListener`, but missed to propagate an `ObservationRegistry`
and other related options.
The `JmsInboundGateway` is worse: it delegated to the `JmsMessageDrivenEndpoint`
* Add `IntegrationObservation.HANDLER` observation to the `MessagingGatewaySupport.send()`
operation: used by the delegate in the `ChannelPublishingJmsMessageListener`
* Expose and propagate observation-related options from `JmsInboundGateway`
and `JmsMessageDrivenEndpoint`
* Expose `observationConvention()` option on the `MessagingGatewaySpec`
and `MessageProducerSpec`
* Remove unused imports
* Do not start a new `RECEIVER` observation if there is already `SERVER` one
* Fix `MessagingGatewaySupport` for `Observation.NOOP` check.
The parent process may still use `ObservationRegistry.NOOP` which sets
`Observation.NOOP` instance into the current context and thread local.
**Cherry-pick to `6.1.x` & `6.0.x`**
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