Add trace events for some of telemetry internal functions

telemetry/telemetry/internal/platform/tracing_controller_backend.py

Index: telemetry/telemetry/internal/platform/tracing_controller_backend.py
diff --git a/telemetry/telemetry/internal/platform/tracing_controller_backend.py b/telemetry/telemetry/internal/platform/tracing_controller_backend.py
index 19c06e53022455bf12393a80ad37680114989791..7f805e8b13f079ce439b94d9035e4d907d51d2b6 100644
--- a/telemetry/telemetry/internal/platform/tracing_controller_backend.py
+++ b/telemetry/telemetry/internal/platform/tracing_controller_backend.py
@@ -195,8 +195,12 @@ class TracingControllerBackend(object):
       for agent in self._active_agents_instances:
         if agent.SupportsExplicitClockSync():
           sync_id = self._GenerateClockSyncId()
-          agent.RecordClockSyncMarker(sync_id,
-                                      self._RecordIssuerClockSyncMarker)
+          with trace_event.trace(
+              'RecordClockSyncMarker',
+              agent=str(agent.__class__.__name__),
+              sync_id=sync_id):
+            agent.RecordClockSyncMarker(sync_id,
+                                        self._RecordIssuerClockSyncMarker)
 
   def IsChromeTracingSupported(self):
     return chrome_tracing_agent.ChromeTracingAgent.IsSupported(