feat: add nzbget_exporter (#368)

- add nzbget_exporter class
- add exporter to nzbget class

Reviewed-on: #368

modules/exporters/templates/nzbget_exporter.service.erb

@@ -0,0 +1,15 @@
+[Unit]
+Description=Prometheus nzbget_exporter
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+User=<%= @user %>
+Group=<%= @group %>
+ExecStart=<%= @exec_path %> --host=<%= @url %>
+ExecReload=/bin/kill -HUP $MAINPID
+KillMode=process
+Restart=always
+
+[Install]
+WantedBy=multi-user.target