Fixes: #9001
The `ObservationPropagationChannelInterceptor` does not propagate an observation properly.
And it fully cannot when the message channel is persistent.
* Deprecate `ObservationPropagationChannelInterceptor` in favor of enabled observation on the channel
and target `MessageHandler` which is a consumer of this channel.
* Remove tests with an `ObservationPropagationChannelInterceptor`
* Mention a correct behavior in the `metrics.adoc` and `ObservationPropagationChannelInterceptor` Javadocs
**Auto-cherry-pick to `6.2.x`**
074d544e94