Metric Persistance

[ozwaldorf]

It'd be neat to be able to have the option to persist the metric state to disk and be able to restore from it on startup, in the event a rust binary is restarted (catching ctrl-c/sigint) or crashes (custom panic hook). Storing the text encoding to disk can work, but there needs to be a way to decode and restore data from the text file. An incremental db approach could also work but I think it'd need more plumbing to be able to hook into when metrics are updated.