Data streaming: run starts: standardise on JSON schema for metadata structure

[rerpha]

https://github.com/ess-dmsc/streaming-data-types/blob/master/schemas/pl72_run_start.fbs#L29 allows for metadata such as the user names and other data which we will need to put in the run start message.

This ticket is to document these so that the keys remain the same between whatever produces them( likely kdae_control) and any downstream consumers such as the journal appender started in #11

acceptance criteria

• A JSON schema is added to this repo that sets the key names of basic fields we would like to the metadata. These should be the ones in the journal XML files.

---

Discussed Planning 08/01/2026 14:36

[Tom-Willemsen]

This may take inspiration from https://github.com/bluesky/event-model which is also a set of JSON schema definitions for how to serialise and deserialise JSON documents with specific keys and meanings.