* Add observation for message channels
* Add observation for message channels
The `MessageChannel.send()` is, essentially, only the point in Spring Integration where we produce a message
and can emit a `PRODUCER` kind span.
* Implement `IntegrationObservation.PRODUCER` infrastructure based on the `MessageSenderContext`
* Implement an observation emission in the `AbstractMessageChannel` based on the mentioned `IntegrationObservation.PRODUCER`
* Build a `MutableMessage.of(message)` to be able to modify message header in the `MessageSenderContext` via tracer `Propagator`
or other tracing injection instrument
* Document which components are instrumented with an `ObservationRegistry`
* Fix language in docs
Co-authored-by: Gary Russell <grussell@vmware.com>
Co-authored-by: Gary Russell <grussell@vmware.com>
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