Add VMMetric and some sample metrics

sdk/lib/profiler/profiler.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart.profiler;
 
 import 'dart:convert';
 
 /// A UserTag can be used to group samples in the Observatory profiler.
 abstract class UserTag {
@@ skipping 59 matching lines @@
 /// and use periods as separators. For example, 'a.b.c'. Uniqueness is only
 /// enforced when a Metric is registered. The name of a metric cannot contain
 /// the slash ('/') character.
 abstract class Metric {
   /// [name] of this metric.
   final String name;
   /// [description] of this metric.
   final String description;
 
   Metric(this.name, this.description) {
-    if (name.contains('/')) {
+    if ((name == 'vm') || name.contains('/')) {
       throw new ArgumentError('Invalid Metric name.');
     }
+
   }
 
   Map _toJSON();
 }
 
 /// A measured value with a min and max. Initial value is min. Value will
 /// be clamped to the interval [min, max].
 class Gauge extends Metric {
   final double min;
   final double max;
@@ skipping 97 matching lines @@
       members.add(metric._toJSON());
     }
     var map = {
       'type': 'MetricList',
       'id': 'metrics',
       'members': members,
     };
     return JSON.encode(map);
   }
 }