add grafana dashboard for SpiceDB

[Panaetius]

SpiceDB exposes various prometheus metrics. Right now we have zero visibility into how SpiceDB behaves. It would be nice to check what each of the exposed metrics to and then add a Dashboard to our Grafana with relevant ones (relevant being ones that gives us a clue on health, load, latency and size/number of relations, that kind of thing).

[olevski]

We should check if somewhere the spice db docs or in their repos there is already specs for a grafana dashboard based on these metrics.