Add label for the DnsWriter in the publishDnsUpdates metrics

This allows grouping metrics based on the DnsWriter. We can already group by
the TLD, but since a TLD can have multiple writers, and since different writers
perform very differently from one another, it could be important to group by
writer as well.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182255398

java/google/registry/dns/PublishDnsUpdatesAction.java

@@ -162,6 +162,7 @@ public final class PublishDnsUpdatesAction implements Runnable, Callable<Void> {
     } finally {
       Duration duration = new Duration(timeAtStart, clock.nowUtc());
       dnsMetrics.recordCommit(
+          dnsWriter,
           commitStatus,
           duration,
           domainsPublished,