1. The elapsed time (`nsd_time_elapsed_seconds`) doesn't have much value
   currently, and doesn't reset when `nsd-control stats` is used, so I would
   probably remove it. However, it could be beneficial to reset this time
   whenever `nsd-control` resets the stats.

I disabled the elapsed time metric, until I point it to the remote-control time value.

2. For the zonestats block: Should the label name "zone" maybe renamed to
   "zonegroup" or something else?

I used zonestats, to copy the name from the config file.

3. Currently, the metrics don't have any HELP text (see [Prometheus Exposition Formats](https://prometheus.io/docs/instrumenting/exposition_formats/#comments-help-text-and-type-information))

Help and type annotations added.

I added the tests for the metrics. One with remote-control enabled, resetting the stats, and basically just sanity checking that the metrics also reset. One with rc disabled just to make sure it works in both cases.
While writing the test, I ran into a segfault when rc was disabled, therefore the fix in f08dec9.