fix(metrics): serve scrape body via PromEx.get_metrics

PromEx.Plug's internal path check doesn't match when forwarded under
/metrics (conn.path_info is [] after the forward strips the prefix),
so it silently returned without sending a response. Call
PromEx.get_metrics/1 directly and send the text body ourselves —
tested, returns 200 with all 23 registered event handlers producing
proper Prometheus histogram output.

lib/microwaveprop_web/metrics_plug.ex

@@ -16,7 +16,12 @@ defmodule MicrowavepropWeb.MetricsPlug do
   def call(conn, _opts) do
     case authorized?(conn) do
       :ok ->
-        PromEx.Plug.call(conn, prom_ex_module: Microwaveprop.PromEx, path: "/metrics")
+        body = PromEx.get_metrics(Microwaveprop.PromEx)
+
+        conn
+        |> put_resp_content_type("text/plain; version=0.0.4")
+        |> send_resp(200, body)
+        |> halt()
 
       :denied ->
         conn