[API] Provide facility to export graphs as time-series data in JSON as well as existing PNG export

Wondering if the API could be of greater use to those looking for reporting integrations if the API provided RRD dumps as time-series JSON in addition to the graph PNGs.

I’m currently achieving this using a third party tool (tbotnz/RRDReST) but it requires a chain of API calls in order to get the required output.

I’m certain this could easily be implemented within LibreNMS, and I’m happy to build this myself and submit a PR - putting this out there to see if anyone else would find this useful.

You can enable additional data store like existing prometheus, opentsdb or influxdb or create your own one by cloning one of them.