Fixes: https://github.com/spring-projects/spring-integration/issues/9743
Spring Integration provides observation for the `MessageChannel`, `MessageHandler`
and `MessageProducerSupport`.
The `SourcePollingChannelAdapter` is missing, and it is that only special endpoint which
deals with `MessageSource` implementations via scheduled tasks in the poller.
Essentially, this endpoint is a start of the flow, but it still is a consumer of data from the source system.
* Add an `Observation` logic to the `SourcePollingChannelAdapter`.
* Divide it into two phases: start (and open scope) when message is received; stop (and close scope) when the whole polling task for a message is done.
We need this separation because of transaction scope for the polling task.
At the same time we don't want to emit an observation for a void polling task.
* Change `MessageReceiverContext` to accept a `handlerType`.
The `MessageHandler` contributes a `handler`.
The new support in the `SourcePollingChannelAdapter` - `message-source`.
And change `MessageProducerSupport` to contribute a `message-producer`
* Verify the single trace is supported for the whole flow (including transaction synchronization) starting from a `SourcePollingChannelAdapter` in a new `SourcePollingChannelAdapterObservationTests`
* Document this new feature
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