prometheus-node-exporter-lua: add target & system to OpenWrt collector

prometheus-node-exporter-lua: add target & system to OpenWrt collector

Before:
node_openwrt_info{revision="r10756+1-7546be6007",model="GL.iNet GL-AR150",id="OpenWrt",board_name="glinet,gl-ar150",release="SNAPSHOT"} 1

After:
node_openwrt_info{revision="r10756+1-7546be6007",target="ath79/generic",board_name="glinet,gl-ar150",id="OpenWrt",model="GL.iNet GL-AR150",release="SNAPSHOT",system="Atheros AR9330 rev 1"} 1

Fixes #9730, replace #9735

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
(cherry picked from commit a14bed1bc959116617b0eb75d2d9a46551a480e6)