* dashboard: change FreeDiskSpace panel to show percentage of used space instead
* dashboard: disable area fill for Cache hit ratio
* dashboard: minor display updates
* dashboard: add panel `Concurrent flushes on disk`
* dashboard: add `Rows ignored` panel
* dashboard: update ChurnRate panel with proper description and additional query over 24h time window
* add panel `Open FDs` for file descriptors metrics;
* add panel `Disk writes/reads` to show the real read/write
load on storage layer;
* add stats panel to show available CPUs, memory and disk space.
* 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
