[telemetry] Make single-benchmark measurements protected and undiscoverable.

tools/telemetry/unittest_data/discoverable_classes/another_discover_dummyclass.py

 # Copyright 2013 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.
 
-"""Another dummy exception subclasses used by core/discover.py's unit tests."""
+"""More dummy exception subclasses used by core/discover.py's unit tests."""
 
 # Import class instead of module explicitly so that inspect.getmembers() returns
 # two Exception subclasses in this current file.
 # Suppress complaints about unable to import class.  The directory path is
 # added at runtime by telemetry test runner.
 #pylint: disable=F0401
 from discoverable_classes.discover_dummyclass import DummyException
 
 
-class AnotherDummyException(DummyException):
+class _PrivateDummyException(DummyException):
   pass
+
+
+class DummyExceptionImpl1(_PrivateDummyException):
+  pass
+
+
+class DummyExceptionImpl2(_PrivateDummyException):
+  pass