[Structured metrics] Add /tools/metrics directory and XML file.

This creates a directory for the structured metrics XML file. The next
CLs will add codegen and presubmits similar to UKM's here too. Some
notes:

Naming:
  In an attempt to use short names, I've called this 'smc' in code,
  for 'structured metrics collection'. I'm not wedded to this though,
  let me know if simply 'structured_metrics' is better.

Ownership:
  For ease of development I've added myself as OWNER, but I don't
  think this is needed in perpetuity. It might be wise to hand off
  ownership at some point, let's talk more about that later on.

  My intention was for the whole Metrics team to inherit ownership of
  this directory. I'm not sure how the owners system works though, so
  let me know if that's not the case.

Types:
  There was some discussion of allowing three kinds of metrics:
  unhashed, globally hashed, and per-user hashed. That will require
  some changes to the XML file format, but I don't think that's
  finalized yet, so let's tackle it in a follow-up CL.

Bug: 1016655
Change-Id: I064bca4ca7bd7b818451f6b49eb400fab43496d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911067Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715454}

tools/metrics/structured/OWNERS

+tby@chromium.org

tools/metrics/structured/structured.xml

+<structured-metrics-configuration>
+<!-- Structured metrics is under development and isn't available for use yet. -->
+
+<event name="TestEvent">
+  <owner>tby@chromium.org</owner>
+  <summary>
+    Event for unit testing, do not use.
+  </summary>
+  <metric name="ValueOne" kind="hashed">
+    <summary>
+      A per-user keyed hashed value.
+    </summary>
+  </metric>
+  <metric name="ValueTwo" kind="hashed">
+    <summary>
+      A per-user keyed hashed value.
+    </summary>
+  </metric>
+  <metric name="ValueThree" kind="unhashed">
+    <summary>
+      An unhashed value, recorded as-is.
+    </summary>
+  </metric>
+</event>
+
+</structured-metrics-configuration>