Improve Grafana dashboard in samples

[javaducky]

Improve upon the currently provided dashboards (samples/).

Another goal for this change would be to align the look-and-feel for the Timescale-backed dashboard (this extension), the xk6-output-influxdb, and xk6-output-prometheus-remote possibly with the output provided by the k6 Cloud App. This would provide a consistent user experience for the varied data sources.

Ideally, this dashboard would be promoted to the Grafana site as the "official" dashboard for displaying k6 metrics backed by TimescaleDB datasources.

This issue may supersede the need for #5.

[micsjo]

Consider optimizing the data model to support a dashboard that works with large volumes of test data. In our case we have created a significant number of materialized views and related policies to keep updated.