Add a sample query for InfluxDB / Flux

[Gowee]

Since InfluxDB 2.0+, the default query language has been changed to Flux which is significantly different than the previous InfluxDB. It takes some extra steps to configure Grafana / InfluxDB to work with InfluxDB, which is inconvenient.

A sample query in Flux should be added to README.

[SomKen]

Bump, an example could be VERY helpful

[gpabdo]

Has anyone figured this out?