Add documentation for monitoring your InfluxDB OSS instance

[russorat]

I see some references scattered around the docs for the /metrics endpoint in OSS, but no content about what it is, why it exists, and what you can do with it.

It would be nice to have a place to send people for monitoring their local influxdb instance that included information about the /metrics endpoint.

I'm thinking the following outline:

Title: Monitoring your InfluxDB Instance

• Enabling/Disabling Prometheus /metrics endpoint
• Securing the endpoint
• Summary of elements found in the endpoint
• Configuring Telegraf to scrape data from the endpoint and send it to cloud
• Configuring an OSS scraper to scrape it
• Configuring a Flux Task with promtheus.scrape to scrape it and store it locally

[jstirnaman]

#3527

[jstirnaman]

75780a9 seems to address a chunk of this. We should review what's left to do.