Add VMMetric and some sample metrics

runtime/vm/json_stream.h

Index: runtime/vm/json_stream.h
diff --git a/runtime/vm/json_stream.h b/runtime/vm/json_stream.h
index 3fc4961e61ae470802d037ae956f9ea35a992906..057fbf973fa5d917e3fc2b5c42c8ea01c7828dc1 100644
--- a/runtime/vm/json_stream.h
+++ b/runtime/vm/json_stream.h
@@ -20,6 +20,7 @@ class JSONArray;
 class JSONObject;
 class Object;
 class SourceBreakpoint;
+class Metric;
 class Zone;
 
 class JSONStream : ValueObject {
@@ -88,6 +89,7 @@ class JSONStream : ValueObject {
   void PrintValue(const Object& o, bool ref = true);
   void PrintValue(SourceBreakpoint* bpt);
   void PrintValue(const DebuggerEvent* event);
+  void PrintValue(Metric* metric);
   void PrintValue(Isolate* isolate, bool ref = true);
 
   void PrintPropertyBool(const char* name, bool b);
@@ -102,6 +104,7 @@ class JSONStream : ValueObject {
 
   void PrintProperty(const char* name, const DebuggerEvent* event);
   void PrintProperty(const char* name, SourceBreakpoint* bpt);
+  void PrintProperty(const char* name, Metric* metric);
   void PrintProperty(const char* name, Isolate* isolate);
   void PrintPropertyName(const char* name);
   void PrintCommaIfNeeded();
@@ -168,6 +171,9 @@ class JSONObject : public ValueObject {
   void AddProperty(const char* name, SourceBreakpoint* bpt) const {
     stream_->PrintProperty(name, bpt);
   }
+  void AddProperty(const char* name, Metric* metric) const {
+    stream_->PrintProperty(name, metric);
+  }
   void AddProperty(const char* name, Isolate* isolate) const {
     stream_->PrintProperty(name, isolate);
   }
@@ -216,6 +222,9 @@ class JSONArray : public ValueObject {
   void AddValue(const DebuggerEvent* event) const {
     stream_->PrintValue(event);
   }
+  void AddValue(Metric* metric) const {
+    stream_->PrintValue(metric);
+  }
   void AddValueF(const char* format, ...) const PRINTF_ATTRIBUTE(2, 3);
 
  private: