* dasbhoard: replace `null` datasources
null datasource value may confuse Grafana and make it drop panel query in some
versions.
* docker: bump grafana image version
* dashboards: add URL variable selector to vmagent dashboard
* dashboards: add new panel `Remote write connection saturation` to vmagent dashboard
* alerts: add new alert for `Remote write connection saturation` panel of vmagent dashboard
* dashboards: add "Logging rate" panel to vmagent dashboard
Alert `TooHighChurnRate24h` suppose to cover cases when churn rate
is low but results in multiple times higher number than total
number of active series.
* dashboard: update single node dashboard
* add panel `Open FDs` for file descriptors metrics;
* add panel `Disk writes/reads` to show the real read/write
load on storage layer;
* add `process_resident_memory_bytes` metric to memory usage panel;
* add stats panel to show available CPUs, memory and disk space;
* rm flags panel since it didn't prove its usefulness.
* alerts: add alert for reaching FDs limit
List contains examples for the alerting rules which might be executed
via `vmalert` to track the health state of VM components. It is assumed
that list will be revised and calibrated for each system individually.
