Start archiving metrics_metadata with builds.

tools/metrics/BUILD.gn

+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+copy("histograms_xml") {
+  sources = [
+    "histograms/histograms.xml",
+  ]
+  outputs = [
+    "$root_out_dir/histograms.xml",
+  ]
+}
+
+copy("actions_xml") {
+  sources = [
+    "actions/actions.xml",
+  ]
+  outputs = [
+    "$root_out_dir/actions.xml",
+  ]
+}
+
+copy("rappor_xml") {
+  sources = [
+    "rappor/rappor.xml",
+  ]
+  outputs = [
+    "$root_out_dir/rappor.xml",
+  ]
+}
+
+group("metrics_metadata") {
+  deps = [
+    ":actions_xml",
+    ":histograms_xml",
+    ":rappor_xml",
+  ]
+}