feat: update grafana dashboard

[0xDones]

Description

• Add datasource variable
• Update panels to use variables instead of default datasource

[KaloyanTanev]

Hey @0xDones , I do not see why we need datasource as a variable, given that we have only prometheus implemented for now?

[0xDones]

@KaloyanTanev this is needed when you are running several DVTs across different kubernetes cluster, which is our case. So we have different Prometheus data sources for each cluster, and this change allow us to dynamically select which data source we want to query

[KaloyanTanev]

@0xDones Hmmm I see now, we will review the PR before the next release, thanks!