Allow Meter ID to be stored in secrets.yaml about rtlamr2mqtt HOT 3 OPEN

Another improvement to this would to also be able to support injecting env like home assistant supports. e.g.

- id: !env_var METER_ID
  protocol: r900
  name: meter_water
  format: "#######.#"
  unit_of_measurement: gal
  icon: mdi:water
  device_class: water
  state_class: total_increasing
  filters:
    - multiply: 0.1

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Either of these methods for pulling Meter IDs out of the primary config would work for me.

Please note: If you want to add Gas or Water to the Energy Dashboard, you MUST change the units to m³. Once added to the energy dashboard, the units can be changed back.

Can you elaborate on this one? I've been wondering why my water units aren't showing up in the Energy Dashboard - they are currently in ft³. Something like this?

1. Change the units to m³ in rtlamr2mqtt, restart rtlamr2mqtt
2. Restart homeassistant, see them on the dashboard
3. Reset units back to ft³ in rtlamr2mqtt, restart rtlamr2mqtt
4. Restart homeassistant, see them on the dashboard

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Here is a link to the start of the conversation where I started getting the Gas section nailed down in the Home Assistant Discord, #energy channel: https://discord.com/channels/330944238910963714/872496321574088765/1041881869958004837

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