[graphite_exporter] Add graphite exporter

[jlpedrosa]

Is your feature request related to a problem ?

No

Describe the solution you'd like.

Have an helm chart to deploy the official graphite_exporter https://github.com/prometheus/graphite_exporter

Describe alternatives you've considered.

Create it manually?

Additional context.

Some systems (TrueNas as an example) don't have the native option to expose prometheus, only graphite, an exporter would help in monitoring such systems.