* dashboard: rename var `datasource` to `ds` for consistency reason
Dasbhoards for cluster version or vmagent operate with datasource variable
named `ds`. For consistency sake we rename this variable in single node version
as well.
* dashboard: add instance variable picker
See dashboard reviews here https://grafana.com/grafana/dashboards/10229/reviews
* dashboard: limit number of buckets in histogram to 12 for vmagent dashboard
* dashboard: bump version requirement in description for single version
* dashboard: drop extra series override for single version
* dashboard: set Y-min to zero for most of panels in vmagent dashboard
`vmagent` Grafana dashboard suppose to provide basic observability over multiple
`vmagent` instances. Dashboard is saved in Grafana export format so it can be easily
imported. It was also integrated into docker-compose environment.
* Slow metrics load panel was removed since it is hard to interpret without
additional metrics and stats;
* Slow inserts panel was updated to display percentage of slow inserts comparing
to total number of inserts to show the real impact.
* The new update introduces new row "Troubleshooting" that
contains panels for churn rate and slow-queries/inserts/loads metrics. This row supposed to be reveal the cause of low performance or other issues;
* CPU panel got `short` units instead of `seconds`;
* Overview row was updated with panel showing bytes-per-datapoint stat;
* Overview row was updated with panel showing free disk space.
The list of changes is following:
* fix Uptime panel column styles according to changes introduced in 6.7 Grafana version
* fix panel `vminsert/Rows per insert` due to metric rename - see #336
* change default datasource to VictoriaMetrics since dashboard now uses MetricsQL for `vminsert/Rows per insert` panel
