add gauge API

lib/camp_api_web/controllers/grist_controller.ex

@@ -0,0 +1,14 @@
+defmodule CampApiWeb.GristController do
+  use CampApiWeb, :controller
+
+  alias CampApi.Grist
+  alias CampApi.Grist.Gauge
+
+  action_fallback CampApiWeb.FallbackController
+
+  def gauge(conn, _params) do
+    gauge = Gauge.get()
+
+    render(conn, :gauge, gauge: gauge)
+  end
+end

lib/camp_api_web/controllers/grist_json.ex

@@ -0,0 +1,7 @@
+defmodule CampApiWeb.GristJSON do
+  alias CampApi.Grist.Gauge
+
+  def gauge(%{gauge: gauge}) do
+    %{gauge: gauge}
+  end
+end