Add readiness check web endpoint

app.rb

@@ -250,6 +250,14 @@ post "/livez/sleep" do
   "ok\n"
 end
 
+get "/readyz" do
+  error 503 unless Ready.instance.ready?
+
+  return Ready.instance.to_json if request.env["HTTP_ACCEPT"] == "application/json"
+
+  Ready.instance.to_s
+end
+
 get "/uuid" do
   n = params.fetch(:n, 1).to_i
   stream do |out|