Parse error in Grafana dashboards due to removed count_scalar

The function count_scalar has been removed as of Prometheus v2.0.0-rc.0, amongst other functions, which has broken a couple of the graphs in this repo. We specifically use the System Overview dashboard, and this has broken the CPU Usage and Saturation Metrics graphs.

How to test: MySQL Overview, ProxySQL Overview, Summary and System Overview dashboards should work with both Prometheus 1.8 and 2.0.