Function stats_document 

pub fn stats_document(registry: &Registry) -> Value

The document emitted by devp stats --json.

Three different vintages of number live here, and the field names say which is which. lifetime has been accumulating since 1.0.0. recent_passes and the bytes_freed inside repositories are only recorded from 1.1.0 onward, so on an upgraded machine they start near zero while lifetime does not — history_starts_at names the version that changed, so a consumer can say so rather than reporting a regression.