Improve InfluxDB monitoring documentation

[vzabawski]

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Current behavior: The documentation that I've found is outdated and gives pretty basic information.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.

[sanderson]

Thanks @vzabawski! I'm transferring this issue to our documentation repo.

Another resource that's available that isn't in your list is here: https://docs.influxdata.com/platform/monitoring/influxdata-platform