Roll infra_libs and gae_ts_mon in luci-py, and add field_specs to all metrics

appengine/third_party/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 58 matching lines @@
     it's not enabled already.
 
 You're done!  You can now use ts_mon metrics exactly as you normally would using
 the infra_libs.ts_mon module. Here's a quick example, but see the
 [timeseries monitoring docs](https://chrome-internal.googlesource.com/infra/infra_internal/+/master/doc/ts_mon.md)
 for more information.
 
     from infra_libs import ts_mon
 
     class MyHandler(webapp2.RequestHandler):
-      goats_teleported = ts_mon.CounterMetric('goats/teleported')
+      goats_teleported = ts_mon.CounterMetric(
+          'goats/teleported',
+          'Number of goats teleported',
+          None)
 
       def get(self):
         count = goat_teleporter.teleport()
         goats_teleported.increment(count)
 
         self.response.write('Teleported %d goats this time' % count)
 
 
 ## Appengine Modules
 
@@ skipping 22 matching lines @@
 
     from infra_libs import ts_mon
 
     # Override default target fields for app-global metrics.
     TARGET_FIELDS = {
         'job_name':  '',  # module name
         'hostname': '',  # version
         'task_num':  0,  # instance ID
     }
 
-    remaining = ts_mon.GaugeMetric('goats/remaining')
-    in_flight = ts_mon.GaugeMetric('goats/in_flight)
+    remaining = ts_mon.GaugeMetric('goats/remaining', '...', None)
+    in_flight = ts_mon.GaugeMetric('goats/in_flight', '...', None)
 
     def set_global_metrics():
       # Query some global resource, e.g. Datastore
       remaining.set(len(goats_to_teleport()), target_fields=TARGET_FIELDS)
       in_flight.set(len(goats_being_teleported()), target_fields=TARGET_FIELDS)
 
     ts_mon.register_global_metrics([remaining, in_flight])
     ts_mon.register_global_metrics_callback('my callback', set_global_metrics)
 
 The registered callback will be called at most once per minute, and
 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.