Define an UID for prometheus datasource

[Marketen]

to be added here: https://github.com/dappnode/DAppNodePackage-DMS/blob/master/grafana/datasource.yml

This could help a lot for identifying prometheus datasource when importing dashboards. Not having an UID defined for a datasoruce is bad practices

nethermind example: ds_prometheus https://github.com/NethermindEth/metrics-infrastructure/blob/master/grafana/provisioning/datasources/datasource.yml