Revise an observation propagation for persisntent message channels

[artembilan]

See more info in: micrometer-metrics/tracing#612.

We cannot store the message with an Observation into database since that one is not Serializable and it cannot be.

We have to revise the logic of the propagation to something like MessageSenderContext and MessageReceiverContext on respective sides of the MessageChannel implementation.
The sender part is already supported as is when we enable an Observation on the channel.
The receiver side is missed so far.
We definitely have to start a new observation on the receiver side since it can be not only in a different thread, but fully different application instance.
And sender observation will be closed any way when that thread finishes its work after storing message into DB.