Create shareable Grafana dashboard

[tvdfly]

I <3 metrics, and already started building out a litefs dash in grafana. Dropping the json into a reusable form somewhere (maybe this repo?), and publishing to https://grafana.com/grafana/dashboards/ would be nice for other devs that use LiteFS and want to observe it's health.

This is mostly a placeholder for me to remember to do this at some point...