Better document new metadata webservice data model
Based on conversations for operations app, expand on these thoughts in the documentation:
Thought more about metadata, and will write this up, but I think this may help clarify how to think about it:
- versions: different versions of one metadata entry this uses the metadata_history (and metadata_id) to track different versions
- epochs: multiple, separate metadata entries representing information that changes over time this uses starttime/endtime to describe when metadata is valid, but there are multiple, separate metadata ids
Readings don't use epochs because they represent measurements at a point in time. Observatories use epochs because the observatory continues to exist, and its information is changing.
So for Observatories and similar epoched metadata, I think we want to be able to edit an epoch (creating a new version without changing starttime/endtime) or start a new epoch (create a new version of the existing by setting an endtime, and creating a separate metadata with the same starttime).