Add support for async, metadata, and counter timeline events to be reported by the embedder

runtime/vm/timeline.cc

Index: runtime/vm/timeline.cc
diff --git a/runtime/vm/timeline.cc b/runtime/vm/timeline.cc
index 959b9adb77ba76d1dcc6e7f0a95040e9037ba434..68850be7e7ad1a2e7e1fe5a431c0000487367a7f 100644
--- a/runtime/vm/timeline.cc
+++ b/runtime/vm/timeline.cc
@@ -379,6 +379,18 @@ void TimelineEvent::End(const char* label,
 }
 
 
+void TimelineEvent::Counter(const char* label, int64_t micros) {
+  Init(kCounter, label);
+  set_timestamp0(micros);
+}
+
+
+void TimelineEvent::Metadata(const char* label, int64_t micros) {
+  Init(kMetadata, label);
+  set_timestamp0(micros);
+}
+
+
 void TimelineEvent::CompleteWithPreSerializedJSON(const char* json) {
   set_pre_serialized_json(true);
   SetNumArguments(1);
@@ -577,6 +589,14 @@ void TimelineEvent::PrintJSON(JSONStream* stream) const {
       obj.AddPropertyF("id", "%" Px64 "", AsyncId());
     }
     break;
+    case kMetadata: {
+      obj.AddProperty("ph", "M");
+    }
+    break;
+    case kCounter: {
+      obj.AddProperty("ph", "C");
+    }
+    break;
     default:
       UNIMPLEMENTED();
   }