Add prometheus metrics

[mozzieongit]

This adds support for Prometheus metrics, exposed using libevent's HTTP server
implementation. It is feature gated with --enable-prometheus-metrics.

The statistics export and socket code is based on code from remote.c.

Here is an excerpt from the exported metrics:

nsd_queries_total{server="0"} 27
nsd_queries_total{server="1"} 22
nsd_queries_by_type_total{type="A"} 42
nsd_queries_by_class_total{class="IN"} 49
nsd_queries_by_opcode_total{opcode="QUERY"} 49
nsd_queries_by_rcode_total{rcode="NOERROR"} 48
nsd_queries_by_transport_total{transport="udp"} 25
nsd_queries_with_edns_total 49
nsd_queries_with_edns_failed_total 0

nsd_connections_total{transport="tcp"} 24
nsd_connections_total{transport="tls6"} 0

nsd_xfr_requests_served_total{xfrtype="AXFR"} 0
nsd_xfr_requests_served_total{xfrtype="IXFR"} 0

nsd_queries_dropped_total 0
nsd_queries_rx_failed_total 0

nsd_answers_tx_failed_total 0
nsd_answers_without_aa_total 0
nsd_answers_truncated_total 0

nsd_time_up_seconds_total 970.392208

nsd_size_db_on_disk_bytes 0
nsd_size_db_in_mem_bytes 0
nsd_size_xfrd_in_mem_bytes 1163464
nsd_size_config_on_disk_bytes 0
nsd_size_config_in_mem_bytes 2208

nsd_zones_primary 1
nsd_zones_secondary 0

nsd_zonestats_examplezone_queries_total 48
nsd_zonestats_examplezone_queries_by_type_total{type="A"} 41

[mozzieongit]

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.