feat(mqtt): add opt-in Home Assistant MQTT discovery

Add an MQTT_HOME_ASSISTANT_DISCOVERY env var that, when enabled, makes
VehicleSubscriber publish HA discovery config payloads (one per entity)
to homeassistant/<component>/teslamate_<car_id>/<object_id>/config on
startup, mirroring the manual configuration documented in
website/docs/integrations/home_assistant.md.

Author: brianmay

config/runtime.exs

@@ -181,7 +181,10 @@ if System.get_env("DISABLE_MQTT") != "true" or config_env() == :test do
     tls: System.get_env("MQTT_TLS") == "true",
     accept_invalid_certs: System.get_env("MQTT_TLS_ACCEPT_INVALID_CERTS") == "true",
     namespace: System.get_env("MQTT_NAMESPACE") |> Util.validate_namespace!(),
-    ipv6: System.get_env("MQTT_IPV6") == "true"
+    ipv6: System.get_env("MQTT_IPV6") == "true",
+    discovery: System.get_env("MQTT_HOME_ASSISTANT_DISCOVERY") == "true",
+    discovery_base_url: System.get_env("MQTT_HOME_ASSISTANT_DISCOVERY_URL"),
+    discovery_prefix: System.get_env("MQTT_HOME_ASSISTANT_DISCOVERY_PREFIX")
 end
 
 if config_env() != :test do

lib/teslamate/mqtt.ex

@@ -16,7 +16,14 @@ defmodule TeslaMate.Mqtt do
     children = [
       {Tortoise311.Connection, connection_config(opts) ++ [client_id: client_id]},
       {Publisher, client_id: client_id},
-      {PubSub, namespace: opts[:namespace]}
+      {PubSub,
+       [
+         namespace: opts[:namespace],
+         discovery: opts[:discovery],
+         discovery_base_url: opts[:discovery_base_url],
+         discovery_prefix: opts[:discovery_prefix]
+       ]
+       |> Enum.reject(fn {_key, value} -> is_nil(value) end)}
     ]
 
     Supervisor.init(children, strategy: :one_for_one)

lib/teslamate/mqtt/pubsub.ex

@@ -12,9 +12,13 @@ defmodule TeslaMate.Mqtt.PubSub do
 
   @impl true
   def init(opts) do
+    subscriber_opts =
+      opts
+      |> Keyword.take([:namespace, :discovery, :discovery_base_url, :discovery_prefix])
+
     children =
       Vehicles.list()
-      |> Enum.map(&{VehicleSubscriber, Keyword.merge(opts, car_id: &1.car.id)})
+      |> Enum.map(&{VehicleSubscriber, Keyword.merge(subscriber_opts, car_id: &1.car.id)})
 
     Supervisor.init(children, strategy: :one_for_one)
   end