Save message attributes in SUEs

[thclark]

Feature request

Use Case

Each service usage event comes with some useful attributes. For example, the ordering key. It would be useful to be able to access these attributes.

Current state

We discard these attributes instead of saving them in the events.

Proposed Solution

Save attributes alongside the data payload.

[cortadocodes]

One thing we'd like to test once this is implemented is if, in a year's time, ordering by the Pub/Sub server set "publish time" and by the "message number" set in the child give the same ordering.