Prometheus datasource for dashboards defaults to webstore-metrics

[meswapnilk]

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.

Ref: open-telemetry/opentelemetry-helm-charts#1479

[puckpuck]

The fear of doing this would be for people who bring their own Prometheus, which happens to include OpenTelemetry metrics. We want to ensure the metrics generated by the demo don't trigger existing alerts or skew existing dashboards.