Consider adding the possibility to save and access metadata

[ricksecretin]

It would be nice to be able to save and access metadata easily in eventuous.

1. Currently a 'null' is saved for metadata when appending events to the eventstore.
   It would be nice if there was a method that you could override like 'AddMetadata' when inheriting from AggregateStore to set the metadata.

2. Currently no access to metadata in event handlers

This would be nice to use the build-in $correlationId of eventStore.
Currently I had to reimplement some of the already implemented classes like aggregateStore to make this possible.

Thanks.

[alexeyzimarev]

Yes, I am working on that already. The upcoming release has a requirement to allow storing and retrieving metadata also because of the distributed tracing feature, which is coming.

[alexeyzimarev]

Btw @ricksecretin if you have any thoughts about #9 please write there.

[alexeyzimarev]

@ricksecretin any thoughts about what to do with the loaded metadata when reading events? The aggregate store can't really do anything with it.

[ricksecretin]

@alexeyzimarev for me personally I don't think it's useful to have access to it when calling the aggregateStore.
I don't directly see a use case where you would need the metadata.

[alexeyzimarev]

Right, that's my thought as well. So far, both ReceivedEvent and ProducedMessage got metadata.

This commit f0afa0a adds traces to the meta for producers. As it uses the AcitivySource API, it works out of the box with OpenTelemetry, although I plan to add some extensions to make it easier. Also, the parent-child relationships will work when upstream and downstream parts are instrumented. You'd need to avoid using tags that start with eventuous as those are reserved. And I forgot to add $correlationId default EventStore meta but I will add it later.