Add VMMetric and some sample metrics

runtime/vm/metrics_test.cc

+// 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.
+
+#include "platform/assert.h"
+
+#include "vm/dart_api_impl.h"
+#include "vm/dart_api_state.h"
+#include "vm/globals.h"
+#include "vm/json_stream.h"
+#include "vm/metrics.h"
+#include "vm/unit_test.h"
+
+namespace dart {
+
+UNIT_TEST_CASE(Metric_Simple) {
+  Isolate* isolate = Isolate::Init(NULL);
+  EXPECT_EQ(isolate, Isolate::Current());
+  VMMetric metric;
+
+  // Initialize metric.
+  metric.Init(Isolate::Current(), "a.b.c", "foobar", VMMetric::kCounter);
+  EXPECT_EQ(0, metric.value());
+  metric.increment();
+  EXPECT_EQ(1, metric.value());
+  metric.set_value(44);
+  EXPECT_EQ(44, metric.value());
+}
+
+class MyMetric : public VMMetric {
+ protected:
+  int64_t Value() const {
+    // 99 bytes.

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

It would be nicer if the unit (and also name and description) could, optionally,
be specified in the subclass declaration instead of the Init call site, since,
as your comment acknowledges, this is the place where you care. I suppose one
could just override Init?

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

Yes. Although overriding is probably more verbose than putting the data at the
Init call. The vast majority of metrics will be in the macro list in the header
which already encodes this information in one place.

+    return 99;
+  }
+
+ public:
+  // Just used for testing.
+  int64_t LeakyValue() const { return Value(); }
+};
+
+UNIT_TEST_CASE(Metric_OnDemand) {
+  Isolate* isolate = Isolate::Init(NULL);
+  EXPECT_EQ(isolate, Isolate::Current());
+  MyMetric metric;
+
+  metric.Init(Isolate::Current(), "a.b.c", "foobar", VMMetric::kByte);
+  // value is still the default value.
+  EXPECT_EQ(0, metric.value());
+  // Call LeakyValue to confirm that Value returns constant 99.
+  EXPECT_EQ(99, metric.LeakyValue());
+
+  // Serialize to JSON.
+  JSONStream js;
+  metric.PrintJSON(&js);
+  const char* json = js.ToCString();
+  EXPECT_STREQ("{\"type\":\"Counter\",\"name\":\"a.b.c\",\"description\":"
+               "\"foobar\",\"unit\":\"byte\",\"id\":\"metrics\\/vm\\/a.b.c\""
+               ",\"value\":99.000000}", json);
+}
+
+}  // namespace dart