* Warn about dropped message in filter
Buy default the `MessageFilter` just drops a discarded message silently.
If a request-reply gateway is used upstream, then it becomes unclear
why the flow sometimes doesn't work.
* Add a waring log ot emit for default behavior.
This still doesn't fix the request-reply problem, but at least it can
give a clue what is going on
* * Mention `nullChannel` variant to ignore even warn
* Fix language in docs
Co-authored-by: Gary Russell <grussell@vmware.com>
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Co-authored-by: Gary Russell <grussell@vmware.com>
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