Prometheus Grafana dashboards update

Changes:
- Singlestat Gauge visualizations updated to latest standalone Gauge visualizations. Singlestat was deprecated in Grafana 7.0 and removed in Grafana 8.0: https://grafana.com/docs/grafana/latest/panels-visualizations/visualizations/stat/
- Legacy Table (old) panels to Table panels. The Table (old) panel uses a deprecated, legacy platform based on AngularJS and will stop working in future releases of Grafana: https://grafana.com/docs/grafana/latest/developers/angular_deprecation/
- Legacy Graph panels to Time Series panels. The Graph (old) panel uses a deprecated, legacy platform based on AngularJS and will stop working in future releases of Grafana: https://grafana.com/docs/grafana/latest/developers/angular_deprecation/