docstr: Test that GetSourceWriter() doesn't discard the loaded event.
code: def testGetSourceWriterLoadsEvent(self):
      """Test that GetSourceWriter() doesn't discard the loaded event."""
      gen = _EventGenerator(self)
      acc = ea.EventAccumulator(gen)
      gen.AddEvent(
          event_pb2.Event(
              wall_time=1,
              step=2,
              file_version="brain.Event:2",
              source_metadata=event_pb2.SourceMetadata(
                  writer="custom_writer"
              ),
          )
      )
  
      self.assertEqual(acc.GetSourceWriter(), "custom_writer")
      acc.Reload()
      self.assertEqual(acc.file_version, 2.0)
  
imports: import os
  import numpy
  import tensorflow
  from tensorboard.backend.event_processing import event_accumulator
  from tensorboard.compat.proto import config_pb2
  from tensorboard.compat.proto import event_pb2
  from tensorboard.compat.proto import graph_pb2
  from tensorboard.compat.proto import meta_graph_pb2
  from tensorboard.compat.proto import summary_pb2
  from tensorboard.plugins.distribution import compressor
  from tensorboard.util import tensor_util
  from tensorboard.util import test_util
  from tensorboard.util import tb_logging
label_desc: Use image node_id 23 for calling the MockingEventAccumulatorTest obj's underlying member method code with example usage: obj.testGetSourceWriterLoadsEvent() without return types