WIP: Grafana 7.2.x+ support #462

Closes #462

This PR adds support for Grafana versions since 7.2.0

For now, only Graphite and Prometheus are tested. Graphite works, Prometheus doesn't (query seems to be changed a bit)

TODO:

test if it works with multiple panels in a dashboard
support the new Prometheus query type (in tsdb-kit)
test other supported datasources
display tsdb-kit errors in the panel (now it looks like nothing happened when datasource query fails)

hastic-zzz / hastic-grafana-app