Support fetching a submission's metadata from the producer side #42
Description
When a submission is made, a producer can add both 'strategic' metadata as well as arbitrary other metadata to the submission.
The strategic metadata can be used when a chunk is picked up to implement fairness: For example, one could set a company_id or user_id and then use the PreferDistinct("company_id", some_other_strategy) to make sure that if there is work to do for multiple companies, both will get picked up.
The arbitrary metadata is just a plain BLOB where info can be stored which inside the consumers we have access to again.
However, sometimes it is useful to read back what metadata (of either kind) was stored in a submission that was created before.
Right now, this is not exposed on the Producer-side API yet. It is possible to look up a submission's status if you have its ID, but this status does not currently include this metadata.
Goal of this issue is to extend that API call so that the metadata is included there as well.