Port metrics test to new framework.

runtime/bin/vmservice/observatory/test/metrics_script.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 allocations_script;
+
+import 'dart:io';
+import 'dart:profiler';
+
+main() {
+  var counter = new Counter('a.b.c', 'description');
+  Metrics.register(counter);
+  counter.value = 1234.5;
+
+  print(''); // Print blank line to signal that we are ready.
+  // Wait until signaled from spawning test.
+  stdin.first.then((_) => exit(0));
+}