Add UMA metric for domain name length

New metric is Net.DNS.RecordParser.DomainNameLength

Bug: 905823
Change-Id: I910ea0bf2629ee2e4663acf998a50debdbed5445
Reviewed-on: https://chromium-review.googlesource.com/c/1340819
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Daniel McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612657}
2 files changed