add unbound-exporter setup tasks and some docs

resources/resolv-dns/files/unbound-exporter.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=Prometheus exporter for Unbound metrics, written in Go with pluggable metric collectors. The metrics exporter converts Unbound metric names to Prometheus metric names and labels by using a set of regular expressions.
+Documentation=https://github.com/letsencrypt/unbound_exporter
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/home/unbound_exporter/go/bin/unbound_exporter \
+    -unbound.ca "" \
+    -unbound.cert "" \
+    -unbound.key "" \
+    -unbound.host "unix:///run/unbound.ctl"
+User=unbound_exporter
+Group=unbound
+
+[Install]
+WantedBy=multi-user.target