Write results of update script to file for debugging

The update script now writes the available instances to a
.update-results* file (where previous runs have "-prev" appended to the
file name). This helps to see how instance availability changes between
runs of the script when debugging overall functionality of the app.

.gitignore

@@ -8,3 +8,6 @@ erl_crash.dump
 *.beam
 /config/*.secret.exs
 .elixir_ls/
+
+# Ignore results from update script
+.update-result*

update.exs

@@ -33,6 +33,7 @@ defmodule Instances do
       end)
 
       add_to_redis(conn, service, result)
+      log_results(service.type, result)
     end
   end
 
@@ -65,6 +66,13 @@ defmodule Instances do
       ])
     end
   end
+
+  def log_results(service_name, results) do
+    {:ok, file} = File.open(".update-results", [:append, {:delayed_write, 100, 20}])
+    IO.write(file, service_name <> ": " <> inspect(results) <> "\n")
+    File.close(file)
+  end
 end
 
+File.rename(".update-results", ".update-results-prev")
 Instances.update("services.json")