Support MetricPoint like mt-gateway does

[D3luxee]

Hi,
i think carbon-relay-ng should support publishing MetricPoints to kafka instead of only MetricData, preferably like mt-gateway implemented it.

Support for MetricPoints would allow the users of carbon-relay-ng and metrictank to ship their data in a more efficient way.

Thanks,
Georg

[Dieterbe]

Agreed.

ideally we share the same library with mt-gateway.
this would be nice to have, but low priority for us (grafana labs)

[D3luxee]

If i find the time i would take a look at it and create a PR.
Should not be that complicated to import the existing publisher from metrictank / mt-gateway and make use of it.