Log synthetic events for trace start/end when the trace finishes (#52937)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52937

Changelog: [internal]

Minor refactor to log the synthetic events for trace start when the trace finishes. This is to simplify future work to implement a sliding window for trace events.

Reviewed By: hoxyq

Differential Revision: D79271692

fbshipit-source-id: 9e923ac36fff850a3aeede7304fb2d721bb9f16c

packages/react-native/ReactCommon/jsinspector-modern/tracing/PerformanceTracer.cpp

@@ -34,7 +34,7 @@ bool PerformanceTracer::startTracing() {
   }
 
   tracingAtomic_ = true;
-
+  currentTraceStartTime_ = HighResTimeStamp::now();
   return true;
 }
 
@@ -60,6 +60,17 @@ std::optional<std::vector<TraceEvent>> PerformanceTracer::stopTracing() {
   // samples will be displayed as empty. We use these events to avoid that.
   // This could happen for non-bridgeless apps, where Performance interface is
   // not supported and no spec-compliant Event Loop implementation.
+
+  events.emplace_back(TraceEvent{
+      .name = "TracingStartedInPage",
+      .cat = "disabled-by-default-devtools.timeline",
+      .ph = 'I',
+      .ts = currentTraceStartTime_,
+      .pid = processId_,
+      .tid = oscompat::getCurrentThreadId(),
+      .args = folly::dynamic::object("data", folly::dynamic::object()),
+  });
+
   events.emplace_back(TraceEvent{
       .name = "ReactNative-TracingStopped",
       .cat = "disabled-by-default-devtools.timeline",

packages/react-native/ReactCommon/jsinspector-modern/tracing/PerformanceTracer.h

@@ -149,7 +149,10 @@ class PerformanceTracer {
    */
   uint32_t performanceMeasureCount_{0};
 
+  HighResTimeStamp currentTraceStartTime_;
+
   std::vector<TraceEvent> buffer_;
+
   /**
    * Protects data members of this class for concurrent access, including
    * the tracingAtomic_, in order to eliminate potential "logic" races.