metrics.json could use more data

Carefully select what should be part of that generic json file. I'm sure we need more data there. But not arbitrary data, as it will grow definitely and its very hard to remove anything from it.

Start by reading metrics.json and reviewing the existing fields and their consumers to understand what the generic file represents. Define which additional data is justified and durable, then update the file with only the selected metrics and verify that existing tooling still handles it correctly.

Written by the indexing model from the issue text.