Add metric for DNS UPDATE latency

Added:
- dns/update_latency, which measures the time since a DNS update was added to the pull queue until that updates is committed to the DnsWriter
- - It doesn't check that after being committed, it was actually published in the DNS.

- dns/publish_queue_delay, which measures how long since the initial insertion to the push queue until a publishDnsUpdate action was handled. It measures both for successes (which is what we care about) and various failures (which are important because the success for that publishDnsUpdate will be > than any of the previous failures)

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185995678

java/google/registry/dns/DnsConstants.java

@@ -30,6 +30,9 @@ public class DnsConstants {
   /** The parameter to use for storing the target name (domain or host name) with the task. */
   public static final String DNS_TARGET_NAME_PARAM = "Target-Name";
 
-  /** The possible values of the {@code DNS_TARGET_NAME_PARAM} parameter. */
+  /** The parameter to use for storing the creation time with the task. */
+  public static final String DNS_TARGET_CREATE_TIME_PARAM = "Create-Time";
+
+  /** The possible values of the {@code DNS_TARGET_TYPE_PARAM} parameter. */
   public enum TargetType { DOMAIN, HOST, ZONE }
 }