* 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 replaces Prometheus to perform scrapes and writes
into VictoriaMetrics installation. Prometheus datasource was
dropped, but its config was reused to feed vmagent.
Change also contains simplification in dashboard propagation
to Grafana container by removing excessive json manipulation
steps.
* 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.
Panels for storage were updated with "bytes-per-datapoint" and "remaining disk size" panels.
The way how regex for column style in Table panel should be applied has changed in 6.7 Grafana version. The change supposed to fix Flags panel column styles accordingly.
