Warn about dropped message in filter (#8579)

* 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>

---------

Co-authored-by: Gary Russell <grussell@vmware.com>

src/reference/asciidoc/filter.adoc

@@ -88,6 +88,10 @@ If you want rejected messages to be routed to a specific channel, provide that r
 ----
 ====
 
+If the `throwExceptionOnRejection == false` and no `discardChannel` is provided, the message is silently dropped and an `o.s.i.filter.MessageFilter` instance just emits a warning log message (starting with version 6.1) about this discarded message.
+To drop the message with no warning in the logs, a `NullChannel` can be configured as the `discardChannel` on the filter.
+The goal of the framework is to not be completely silent, by default, requiring an explicit option to be set, if that is the desired behavior.
+
 See also <<./handler-advice.adoc#advising-filters,Advising Filters>>.
 
 NOTE: Message filters are commonly used in conjunction with a publish-subscribe channel.

src/reference/asciidoc/whats-new.adoc

@@ -30,6 +30,8 @@ See <<./zip.adoc#zip,Zip Support>>  for more information.
  - Added support for transforming to/from Protocol Buffers.
  See <<./transformer.adoc#Protobuf-transformers, Protocol Buffers Transformers>> for more information.
 
+ - The `MessageFilter` now emits a warning into logs when message is silently discarded and dropped.
+See <<./filter.adoc#filter, Filter>> for more information.
 
 [[x6.1-web-sockets]]
 === Web Sockets Changes