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

runtime/vm/timeline.h

Index: runtime/vm/timeline.h
diff --git a/runtime/vm/timeline.h b/runtime/vm/timeline.h
index 0e0cf734ba3436f9f75d1bf7e5833e8366bced77..9196ee08e81e8ec8f14a99f46eba34becf71428c 100644
--- a/runtime/vm/timeline.h
+++ b/runtime/vm/timeline.h
@@ -128,6 +128,8 @@ class TimelineEvent {
     kAsyncBegin,
     kAsyncInstant,
     kAsyncEnd,
+    kCounter,
+    kMetadata,
     kNumEventTypes,
   };
 
@@ -169,6 +171,12 @@ class TimelineEvent {
   void End(const char* label,
            int64_t micros = OS::GetCurrentMonotonicMicros());
 
+  void Counter(const char* label,
+               int64_t micros = OS::GetCurrentMonotonicMicros());
+
+  void Metadata(const char* label,
+                int64_t micros = OS::GetCurrentMonotonicMicros());
+
   // Completes this event with pre-serialized JSON. Copies |json|.
   void CompleteWithPreSerializedJSON(const char* json);