Fix dialyzer warning

Author: mtrudel

test/support/telemetry_helpers.ex

@@ -11,7 +11,10 @@ defmodule TelemetryHelpers do
 
   def attach_all_events(plug_or_websock) do
     ref = make_ref()
-    :telemetry.attach_many(ref, @events, &__MODULE__.handle_event/4, {self(), plug_or_websock})
+
+    _ =
+      :telemetry.attach_many(ref, @events, &__MODULE__.handle_event/4, {self(), plug_or_websock})
+
     fn -> :telemetry.detach(ref) end
   end