Add metrics for async batch operation processing

We want to know how long it's actually taking to process asynchronous
contact/host deletions and DNS refreshes on host renames. This adds
instrumentation. Five metrics are recorded as follows:

* An incrementable metric for each async task processed (split out by
  type of task and result).
* Two event metrics for processing time between when a task is enqueued
  and when it is processed -- tracked separately for contact/host
  deletion and DNS refresh on host rename.
* Two event metrics for batch size every time the two mapreduces are
  run (this is usually 0). Tracked separately for contact/host deletion
  and DNS refresh on host rename.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=157001310

javatests/google/registry/flows/host/HostUpdateFlowTest.java

@@ -192,7 +192,9 @@ public class HostUpdateFlowTest extends ResourceFlowTestCase<HostUpdateFlow, Hos
     // Task enqueued to change the NS record of the referencing domain via mapreduce.
     assertTasksEnqueued(
         QUEUE_ASYNC_HOST_RENAME,
-        new TaskMatcher().param("hostKey", Key.create(renamedHost).getString()));
+        new TaskMatcher()
+            .param("hostKey", Key.create(renamedHost).getString())
+            .param("requestedTime", clock.nowUtc().toString()));
   }
 
   @Test