Use keyword list in Logger.Backend

Instead of taking a string as the log group, take a keyword list with a
single key, for possible extension later.

lib/appsignal/logger/backend.ex

@@ -1,20 +1,16 @@
 defmodule Appsignal.Logger.Backend do
   @behaviour :gen_event
 
-  def init({__MODULE__, group}) do
-    {:ok, group}
+  def init({__MODULE__, options}) do
+    {:ok, Keyword.merge([group: "app"], options)}
   end
 
-  def init(_) do
-    {:ok, "app"}
+  def handle_event({level, _gl, {Logger, message, _timestamp, metadata}}, options) do
+    Appsignal.Logger.log(level, options[:group], IO.chardata_to_string(message), Enum.into(metadata, %{}))
+    {:ok, options}
   end
 
-  def handle_event({level, _gl, {Logger, message, _timestamp, metadata}}, group) do
-    Appsignal.Logger.log(level, group, IO.chardata_to_string(message), Enum.into(metadata, %{}))
-    {:ok, group}
-  end
-
-  def handle_call(_messsage, group) do
-    {:ok, nil, group}
+  def handle_call(_messsage, options) do
+    {:ok, nil, options}
   end
 end

test/appsignal/logger/backend_test.exs

@@ -31,7 +31,7 @@ defmodule Appsignal.Logger.BackendTest do
           ]
         }
       },
-      "phoenix"
+      [group: "phoenix"]
     )
 
     assert [{"phoenix", 3, "foo bar baz", _}] = Appsignal.Test.Nif.get!(:log)