gae_ts_mon: instrument Cloud Endpoint methods

appengine_module/gae_ts_mon/README.md

 # Setting up timeseries monitoring on App Engine.
 
 1.  Symlink this directory into your appengine app.
 
         cd infra/appengine/myapp
         ln -s ../../appengine_module/gae_ts_mon .
 
 1.  Add the scheduled task to your `cron.yaml` file.  Create it if you don't
     have one already.
 
@@ skipping 14 matching lines @@
 
         [...]
 
         app = webapp2.WSGIApplication(my_handlers)
         gae_ts_mon.initialize(app, cron_module='<cron_module_name>')
 
     You must do this in every top-level request handler that's listed in your
     app.yaml to ensure metrics are registered no matter which type of request
     an instance receives first.
 
+    If your app does not contain a `webapp2.WSGIApplication` instance
+    (e.g. it's a Cloud Endpoints only app), then pass `None` as the
+    first argument to `gae_ts_mon.initialize`.
+
     The `gae_ts_mon.initialize` method takes a few optional parameters:
      - `cron_module` (str, default='default'): if you specified a custom
        module for the cron job, you must specify it in every call to
        `initialize`.
      - `is_enabled_fn` (function with no arguments returning `bool`):
        a callback to enable/disable sending the actual metrics. Default: `None`
        which is equivalent to `lambda: True`. The callback is called on every
        metrics flush, and takes effect immediately. Make sure the callback is
        efficient, or it will slow down your requests.
 
+1.  Instrument all Cloud Endpoint methods if you have any by adding a decorator:
+
+        @gae_ts_mon.instrument_endpoint()
+        @endpoints.method(...)
+        def your_method(self, request):
+          ...
+
 1.  Give your app's service account permission to send metrics to the API.  You
     can find the name of your service account on the `Permissions` page of your
     project in the cloud console - it'll look something like
     `app-id@appspot.gserviceaccount.com`.  Or search the default module's logs
     for `"Initializing with service account"`.  Add it as a "Publisher" of the
     "monacq" PubSub topic in the
     [chrome-infra-mon-pubsub project](https://pantheon.corp.google.com/project/chrome-infra-mon-pubsub/cloudpubsub/topicList)
     by selecting it from the list and clicking "Permissions". If you see an
     error "You do not have viewing permissions for the selected resource.", then
     please ask pgervais@chromium.org (AMER) or sergiyb@chromium.org (EMEA) to do
@@ skipping 68 matching lines @@
 only one instance will be running it at a time. A global metric is
 then cleared the moment it is sent.  Thus, global metrics will be sent
 at the correct intervals, regardless of the number of instances the
 app is currently running.
 
 Note also the use of `target_fields` parameter: it overrides the
 default target fields which would otherwise distinguish the metric per
 module, version, or instance ID. Using `target_fields` in regular,
 "local" metrics is not allowed, as it would result in errors on the
 monitoring endpoint, and loss of data.