[demo] Prometheus datasource for dashboards defaults to webstore-metrics

[Pionerd]

Currently the Grafana dashboards default to the (in my humble opinion) arbitrarily named datasource webstore-metrics. Especially when "bringing your own observability" I would expect this to default to prometheus.

Given not everyone will BYO prometheus, the reason for a unique name is probably to be able to coexist next to another instance.

So moving forward, I suspect the best way is to update the dashboards with a configurable datasource? If agreed, I'll make a PR.