* Initial support for Micrometer Observation
* Add respective Observation dependencies
* Refactor an `AbstractMessageHandler` logic for potential Observation hooks
* Introduce an `ObservationPropagationChannelInterceptor` to propagate
an `Observation` from one thread to another through message channels
* Adds an example of propagation
* Fixed the user code and receiving spans
* * Clean up for Tracing unit test
* Make `micrometer-observation` as an `api` dep - non-optional for direct API usage
Co-authored-by: Marcin Grzejszczak <marcin@grzejszczak.pl>
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