We should report our own DB size -- row counts, disk size, etc. -- as a crucial part of our own health. We need to know if we're rendering users' handsets unusable by accruing thousands of sessions or hundreds of environments.
There does not appear to be an entry for database information in the document schema as of Version 3 (https://docs.services.mozilla.com/healthreport/index.html#payload-format). How would one go about adding to the schema? The weekly meetings seem to imply that the addition of any probes should be discussed there.
Nope, it's not yet defined. Best starting point is to discuss something with gps after looking at our schema and what Android lets you find out. New probes need discussion if they leak user info, mostly. Health reporting for health report itself shouldn't be contentious.