Add --print-metrics vm flag for printing metrics to console

runtime/vm/metrics.h

Index: runtime/vm/metrics.h
diff --git a/runtime/vm/metrics.h b/runtime/vm/metrics.h
index ab588811600fd4690b739b3a836b193a61d32e28..03cead211bbb2cd7a0030a5fad3a2b73a653710e 100644
--- a/runtime/vm/metrics.h
+++ b/runtime/vm/metrics.h
@@ -35,6 +35,8 @@ class Metric {
 
   static void InitOnce();
 
+  static void Cleanup();
+
   // Initialize and register a metric for an isolate.
   void Init(Isolate* isolate,
             const char* name,
@@ -50,6 +52,12 @@ class Metric {
 
   void PrintJSON(JSONStream* stream);
 
+  // Returns a zone allocated string.
+  static char* ValueToString(int64_t value, Unit unit);
+
+  // Returns a zone allocated string.
+  char* ToString();
+
   int64_t value() const { return value_; }
   void set_value(int64_t value) { value_ = value; }