Include 'fixedId' key when printing service ids

runtime/vm/metrics.cc

Index: runtime/vm/metrics.cc
diff --git a/runtime/vm/metrics.cc b/runtime/vm/metrics.cc
index 2e768b5db884f83218aae0b0dd9b1ab9a1db9bdc..0e04e6bc1df648db2633d81013fa65279d824ea3 100644
--- a/runtime/vm/metrics.cc
+++ b/runtime/vm/metrics.cc
@@ -82,9 +82,9 @@ void Metric::PrintJSON(JSONStream* stream) {
   obj.AddProperty("description", description_);
   obj.AddProperty("unit", UnitString(unit()));
   if (isolate_ == NULL) {
-    obj.AddPropertyF("id", "vm/metrics/%s", name_);
+    obj.AddFixedServiceId("vm/metrics/%s", name_);
   } else {
-    obj.AddPropertyF("id", "metrics/native/%s", name_);
+    obj.AddFixedServiceId("metrics/native/%s", name_);
   }
   // TODO(johnmccutchan): Overflow?
   double value_as_double = static_cast<double>(Value());