prometheus-node-exporter: Support extra options.

There are plenty of options supported that the Guix configuration record
doesn't help you with, so add this field to allow users to do their own thing.

* gnu/services/monitoring.scm (<prometheus-node-exporter-configuration>): Add
extra-options field.
(prometheus-node-exporter-shepherd-service): Handle the extra options.
* doc/guix.texi (Prometheus Node Exporter Service): Document this.