Add VMMetric and some sample metrics

tests/lib/profiler/metrics_test.dart

Index: tests/lib/profiler/metrics_test.dart
diff --git a/tests/lib/profiler/metrics_test.dart b/tests/lib/profiler/metrics_test.dart
index 69be4460ad3ee3acb40d8120ec602ec81d6cfd21..4dbf642320cc425a047b43b84ca954eb1c5c9a97 100644
--- a/tests/lib/profiler/metrics_test.dart
+++ b/tests/lib/profiler/metrics_test.dart
@@ -99,6 +99,9 @@ testBadName() {
   Expect.throws(() {
     var counter = new Counter('a.b/c', 'description');
   });
+  Expect.throws(() {
+    var counter = new Counter('vm', 'description');

[koda, 2014/08/13 22:43:00]

Could you also add a test to observatory/test that successfully requests a VM
metric?

[Cutch, 2014/08/14 20:56:19]

Done.

+  });
 }
 
 main() {