* Add missing serializers (and rename classes because metadata is a word)
* Add test instances for journal and snapshot support
* Method to add metadata to existing snapshotmetadata
* Better error message if replicated stream does not have metadata
* Snapshot store tck support for metadata
* Docs for tck coverage of AA metadata
Updated stream-composition.md to use Flow.fromSinkAndSourceCoupledMat or Flow.fromSinkAndSourceCoupled rather than the deprecated CoupledTerminationFlow.fromSinkAndSource.
Fixes#28769
Use case for this is if you have a sequence of elements that has been
partitioned across multiple streams, and you want to merge them back
together in order. It will typically be used in combination with
`zipWithIndex` to define the index for the sequence, followed by a
`Partition`, followed by the processing of different substreams with
different flows (each flow emitting exactly one output for each input),
and then merging with this stage, using the index from `zipWithIndex`.
A more concrete use case is if you're consuming messages from a message
broker, and you have a flow that you wish to apply to some messages, but
not others, you can partition the message stream according to which
should be processed by the flow and which should bypass it, and then
bring the elements back together acknowledgement. If an ordinary merge
was used rather than this, the messages that bypass the processing flow
would likely overtake the messages going through the processing flow,
and the result would be out of order offset acknowledgement which would
lead to dropping messages on failure.
I've included a minimal version of the above example in the documentation.
New type StatusReply simplifies the very common use case of replying to a request with either a successful reply or an error reply which can be repetitive to define for every actor, with the additional overhead of having to make sure each such sealed top type + 2 concrete reply classes has working serialization.
