Add VMMetric and some sample metrics

runtime/bin/vmservice/observatory/test/vm_metrics_test.dart

 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
 import 'dart:profiler';
 
 void script() {
   var counter = new Counter('a.b.c', 'description');
   Metrics.register(counter);
   counter.value = 1234.5;
 }
 
 var tests = [
 
 (Isolate isolate) =>
-  isolate.get('metrics').then((ServiceMap metrics) {
+  isolate.get('metrics/vm').then((ServiceMap metrics) {
     expect(metrics['type'], equals('MetricList'));
     var members = metrics['members'];
     expect(members, isList);
-    expect(members.length, equals(1));
-    var counter = members[0];
-    expect(counter['name'], equals('a.b.c'));
-    expect(counter['value'], equals(1234.5));
+    expect(members.length, greaterThan(1));
+    var foundOldHeapCapacity = members.any((m) =>
+        m['name'] == 'heap.old.capacity');
+    expect(foundOldHeapCapacity, equals(true));
 }),
 
 (Isolate isolate) =>
-  isolate.get('metrics/a.b.c').then((ServiceMap counter) {
-    expect(counter['name'], equals('a.b.c'));
-    expect(counter['value'], equals(1234.5));
+  isolate.get('metrics/vm/heap.old.used').then((ServiceMap counter) {
+    expect(counter['type'], equals('Counter'));
+    expect(counter['name'], equals('heap.old.used'));
 }),
 
 (Isolate isolate) =>
-  isolate.get('metrics/a.b.d').then((DartError err) {
+  isolate.get('metrics/vm/doesnotexist').then((DartError err) {
     expect(err is DartError, isTrue);
 }),
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);