prometheus-node-exporter-lua: add realtek-poe exporter

prometheus-node-exporter-lua: add realtek-poe exporter

exposes PoE metrics obtained from realtek-poe

 # HELP realtek_poe_switch_info information about the poe controller
 # TYPE realtek_poe_switch_info gauge
 realtek_poe_switch_info{mcu="ST Micro ST32F100 Microcontroller",firmware="v22.4"} 1

 # HELP realtek_poe_switch_budget_watts overall power budget
 # TYPE realtek_poe_switch_budget_watts gauge
 realtek_poe_switch_budget_watts 77

 # HELP realtek_poe_switch_consumption_watts overall power consumption
 # TYPE realtek_poe_switch_consumption_watts gauge
 realtek_poe_switch_consumption_watts 5

 # HELP realtek_poe_port_priority poe priority of port
 # TYPE realtek_poe_port_priority gauge
 realtek_poe_port_priority{device="lan1"} 1
 realtek_poe_port_priority{device="lan2"} 1
 [...]

 # HELP realtek_poe_port_consumption_watts per port power consumption
 # TYPE realtek_poe_port_consumption_watts gauge
 realtek_poe_port_consumption_watts{device="lan1"} 0
 realtek_poe_port_consumption_watts{device="lan2"} 0
 [...]

 # HELP realtek_poe_port_state per port poe state
 # TYPE realtek_poe_port_state gauge
 realtek_poe_port_state{device="lan1",state="Disabled"} 0
 realtek_poe_port_state{device="lan1",state="Searching"} 1
 [...]
 (states: Disabled, Searching, Delivering power, Fault, Other fault, Requesting power)

 # HELP realtek_poe_port_mode per port poe mode
 # TYPE realtek_poe_port_mode gauge
 realtek_poe_port_mode{device="lan1",mode="PoE"} 0
 realtek_poe_port_mode{device="lan1",mode="PoE+"} 1
 [...]
 (modes: PoE, Legacy, pre-PoE+, PoE+)

Signed-off-by: Gregor Michels <hirnpfirsich@brainpeach.de>