Improve InfluxDB monitoring documentation

__Proposal:__
Update the documentation that describes InfluxDB monitoring with the most important metrics. This will be useful for new users.

__Current behavior:__
The documentation that I've found is outdated and gives pretty basic information.
- https://github.com/influxdata/influxdb/blob/1.7/monitor/README.md
- https://www.influxdata.com/blog/how-to-use-the-show-stats-command-and-the-_internal-database-to-monitor-influxdb/
- https://docs.influxdata.com/influxdb/v1.7/administration/server_monitoring/#useful-performance-metrics-commands

__Desired behavior:__
It would be nice if the most crucial metrics are provided in the documentation. Maybe some examples and approaches described.