prometheus-node-exporter: Enable the textfile collector.

* gnu/services/monitoring.scm (<prometheus-node-exporter-configuration>): Add
textfile-directory.
(prometheus-node-exporter-textfile-directory,
prometheus-node-exporter-activation): New procedures.
(prometheus-node-exporter-shepherd-service): Pass
--collector.textfile.directoryto the service.
(prometheus-node-exporter-service-type): Extend the activation service type.
* doc/guix.texi (Prometheus Node Exporter Service): Document.