[node_exporter] Add textcollector in order to fetch more metrics

[deajan]

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Is your feature request related to a problem? Please describe.
I use node_exporter to monitor my opnsense firewall.
I want to know when a gateway is down.

Describe the solution you'd like
So far, I can run a script that will create a file in let's say /var/lib/node_exporter/textcollector/gateways.prom with the following content:

# TYPE opnsense_gateway gauge
# HELP opnsense_gateway zero if gateway is okay, 1 otherwise
opnsense_gateway{gateway_name="wan1"} 0
opnsense_gateway{gateway_name="wan2"} 1

Enabling text collector for node_exporter would allow to pick up these metrics.

Describe alternatives you've considered
There's SNMP, but that's alot of work.

Additional context

Perhaps there should exist some kind of repository with basic scripts that could provide more prometheus metrics.
Willing to provide my gateway script here.