Enhancement request, not issue: Add support for `nsd`

[james-stevens]

nsd and unbound share the same parent (NLNetLabs), so share a lot of the same mechanism for accessing metrics. Therefore, I would think it should be relatively easy to expand this to also support nsd.

From what I can see, a lot of the metrics reported are also common. Here's a list from an open source nsd exporter

https://github.com/optix2000/nsd_exporter/blob/master/config/config.yaml

I'm not sure how much that package is actively maintained as the last release was 2021, a few metrics are missing, it doesn't support unix sockets and I had significantly more problems getting it working than yours - hence, I'd really prefer to see nsd added here.