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/1340819Reviewed-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}

net/dns/dns_response.cc

@@ -11,6 +11,7 @@
 
 #include "base/big_endian.h"
 #include "base/logging.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/numerics/safe_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/sys_byteorder.h"
@@ -211,6 +212,8 @@ bool DnsRecordParser::ReadRecord(DnsResourceRecord* out) {
       reader.ReadU16(&rdlen) &&
       reader.ReadPiece(&out->rdata, rdlen)) {
     cur_ = reader.ptr();
+    UMA_HISTOGRAM_COUNTS_10000("Net.DNS.RecordParser.DomainNameLength",
+                               out->name.length());
     return true;
   }
   return false;

tools/metrics/histograms/histograms.xml

@@ -57203,6 +57203,14 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Net.DNS.RecordParser.DomainNameLength" units="octets">
+  <owner>dmcardle@chromium.org</owner>
+  <owner>davidben@chromium.org</owner>
+  <summary>
+    Length of domain name parsed by DnsRecordParser::ReadName().
+  </summary>
+</histogram>
+
 <histogram name="Net.DNS.ResolveCategory" enum="ResolutionCategory">
   <owner>pauljensen@chromium.org</owner>
   <owner>mef@chromium.org</owner>