Add trace events for some of telemetry internal functions

telemetry/telemetry/internal/backends/app_backend.py

 # Copyright 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+from py_trace_event import trace_event
+
 
 class AppBackend(object):
+
+  __metaclass__ = trace_event.TracedMetaClass
+
   def __init__(self, app_type, platform_backend):
     super(AppBackend, self).__init__()
     self._app = None
     self._app_type = app_type
     self._platform_backend = platform_backend
 
   def __del__(self):
     self.Close()
 
   def SetApp(self, app):
@@ skipping 31 matching lines @@
     raise NotImplementedError
 
   def GetStandardOutput(self):
     raise NotImplementedError
 
   def GetStackTrace(self):
     raise NotImplementedError
 
   def GetMostRecentMinidumpPath(self):
     raise NotImplementedError