Add a prometheus exporter for Modem Manager Signal

[macjl]

Maintainer: Jean-Laurent Girod
Compile tested: No compile test done
Run tested: ath79/generic, GL.inet GL-X300B, OpenWrt 23.05.5

Description:
Add a prometheus exporter that rely on modemmanager to provide signal strength informations.

[feckert]

Please use one commit for this change, and also change the commit header to prometheus-node-exporter-lua: add modemmanager exporter.
How does the exporter behave if there is more than one modem in the system?

Please have in mind, that if the modem is reattached The object ID in the ModemMananager Service is increment by one on every reattach. Is this handled by your code correctly?

[macjl]

Please use one commit for this change, and also change the commit header to prometheus-node-exporter-lua: add modemmanager exporter.

This is done !

How does the exporter behave if there is more than one modem in the system?
Please have in mind, that if the modem is reattached The object ID in the ModemMananager Service is increment by one on every reattach. Is this handled by your code correctly?

I don’t found any informations about the output of mmcli command when there are 2 modems, as I just have one modem to test it. Is there a way I could simulate a second one ?