Create Tasks.Tags endpoint and Cron job

appengine/swarming/swarming_rpcs.py

Index: appengine/swarming/swarming_rpcs.py
diff --git a/appengine/swarming/swarming_rpcs.py b/appengine/swarming/swarming_rpcs.py
index cb92dfd82cd3321fc95bb558a10123f5dba171fc..6c290eabf170b66a50fa05f919ff28f70da9a78c 100644
--- a/appengine/swarming/swarming_rpcs.py
+++ b/appengine/swarming/swarming_rpcs.py
@@ -302,6 +302,13 @@ class TasksCount(messages.Message):
   now = message_types.DateTimeField(2)
 
 
+class TasksTags(messages.Message):
+  """Returns all the tags and tag possibilities in the fleet."""
+  tasks_tags = messages.MessageField(StringListPair, 1, repeated=True)
+  # Time at which this summary was calculated.
+  ts = message_types.DateTimeField(2)
+
+
 class TaskRequestMetadata(messages.Message):
   """Provides the ID of the requested TaskRequest."""
   request = messages.MessageField(TaskRequest, 1)