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/HostUpdateFlow.java

@@ -272,7 +272,7 @@ public final class HostUpdateFlow implements TransactionalFlow {
       }
       // We must also enqueue updates for all domains that use this host as their nameserver so
       // that their NS records can be updated to point at the new name.
-      asyncFlowEnqueuer.enqueueAsyncDnsRefresh(existingHost);
+      asyncFlowEnqueuer.enqueueAsyncDnsRefresh(existingHost, ofy().getTransactionTime());
     }
   }