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.

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

java/google/registry/flows/host/HostDeleteFlow.java

@@ -108,7 +108,8 @@ public final class HostDeleteFlow implements TransactionalFlow {
               : existingHost;
       verifyResourceOwnership(clientId, owningResource);
     }
-    asyncFlowEnqueuer.enqueueAsyncDelete(existingHost, clientId, trid, isSuperuser);
+    asyncFlowEnqueuer.enqueueAsyncDelete(
+        existingHost, ofy().getTransactionTime(), clientId, trid, isSuperuser);
     HostResource newHost =
         existingHost.asBuilder().addStatusValue(StatusValue.PENDING_DELETE).build();
     historyBuilder