Add max post-gc heap usage metrics

runtime/vm/metrics.cc

Index: runtime/vm/metrics.cc
diff --git a/runtime/vm/metrics.cc b/runtime/vm/metrics.cc
index 274a3fd179f5025f1a90f1adf032497eae0ee788..0e15c0fb84f38ab9ed8dc3fc26ee7282ba69c0a5 100644
--- a/runtime/vm/metrics.cc
+++ b/runtime/vm/metrics.cc
@@ -276,6 +276,12 @@ int64_t MetricHeapNewExternal::Value() const {
 }
 
 
+int64_t MetricHeapUsed::Value() const {
+  ASSERT(isolate() == Isolate::Current());
+  return isolate()->heap()->UsedInWords(Heap::kNew) * kWordSize +
+         isolate()->heap()->UsedInWords(Heap::kOld) * kWordSize;
+}
+
 int64_t MetricIsolateCount::Value() const {
   return Isolate::IsolateListLength();
 }