Add macos-battery-exporter script and launchd config

It reads and exports battery metrics for Prometheus.

bin/macos_battery_exporter

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+#! /usr/bin/env ruby
+# frozen_string_literal: true
+
+require 'json'
+
+class MacOSBatteryExporter
+  def export
+    puts metrics
+  end
+
+  def metrics
+    <<~METRICS
+      # HELP node_battery_charge_percent Battery charge percent
+      # TYPE node_battery_charge_percent gauge
+      node_battery_charge_percent{serial="#{battery_serial_number}"} #{charge_percentage}
+      # HELP node_battery_charge_ampere Battery charge ampere
+      # TYPE node_battery_charge_ampere gauge
+      node_battery_charge_ampere{serial="#{battery_serial_number}"} #{charge_info['sppower_battery_current_capacity'] / 1000.0}
+      # HELP node_battery_full_charge_ampere Battery full charge capacity
+      # TYPE node_battery_full_charge_ampere gauge
+      node_battery_full_charge_ampere{serial="#{battery_serial_number}"} #{charge_info['sppower_battery_max_capacity'] / 1000.0}
+      # HELP node_battery_current_flow_ampere Current flow of ampere in (+) or out (-) of battery
+      # TYPE node_battery_current_flow_ampere gauge
+      node_battery_current_flow_ampere{serial="#{battery_serial_number}"} #{battery_info['sppower_current_amperage'] / 1000.0}
+      # HELP node_battery_charger_connected Is charger connected?
+      # TYPE node_battery_charger_connected gauge
+      node_battery_charger_connected{serial="#{battery_serial_number}"} #{ac_charger_info['sppower_battery_charger_connected'] == 'TRUE' ? 1.0 : 0.0}
+      # HELP node_battery_is_charging Is charger connected?
+      # TYPE node_battery_is_charging gauge
+      node_battery_is_charging{serial="#{battery_serial_number}"} #{ac_charger_info['sppower_battery_is_charging'] == 'TRUE' ? 1.0 : 0.0}
+      # HELP node_battery_charger_watts Watts provided by charger
+      # TYPE node_battery_charger_watts gauge
+      node_battery_charger_watts{serial="#{battery_serial_number}"} #{ac_charger_info['sppower_ac_charger_watts'].to_f}
+      # HELP node_battery_fully_charged Is battery fully charged?
+      # TYPE node_battery_fully_charged gauge
+      node_battery_fully_charged{serial="#{battery_serial_number}"} #{ac_charger_info['sppower_battery_fully_charged'] == 'TRUE' ? 1.0 : 0.0}
+      # HELP node_battery_cycle_count Battery cycle count
+      # TYPE node_battery_cycle_count counter
+      node_battery_cycle_count{serial="#{battery_serial_number}"} #{health_info['sppower_battery_cycle_count']}
+    METRICS
+  end
+
+  def charge_percentage
+    @charge_percentage ||=
+      `pmset -g batt | grep -Eo '\\d+%' | cut -d% -f1`.strip
+  end
+
+  def battery_serial_number
+    battery_info.dig(
+      'sppower_battery_model_info',
+      'sppower_battery_serial_number'
+    )
+  end
+
+  def charge_info
+    @charge_info ||= battery_info['sppower_battery_charge_info']
+  end
+
+  def health_info
+    @health_info ||= battery_info['sppower_battery_health_info']
+  end
+
+  def battery_info
+    @battery_info ||= power_data&.find do |v|
+      v['_name'] == 'spbattery_information'
+    end
+  end
+
+  def ac_charger_info
+    @ac_charger_info ||= power_data&.find do |v|
+      v['_name'] == 'sppower_ac_charger_information'
+    end
+  end
+
+  def power_data
+    @power_data ||= JSON.parse(`system_profiler SPPowerDataType -json`)
+                        &.[]('SPPowerDataType')
+  end
+end
+
+MacOSBatteryExporter.new.export if __FILE__ == $PROGRAM_NAME