docstr: Test that FirstEventTimestamp() returns wall_time of the first
  event.
code: def testFirstEventTimestamp(self):
      """Test that FirstEventTimestamp() returns wall_time of the first
      event."""
      gen = _EventGenerator(self)
      acc = ea.EventAccumulator(gen)
      gen.AddEvent(
          event_pb2.Event(
              wall_time=10, step=20, file_version="brain.Event:2"
          )
      )
      gen.AddScalar("s1", wall_time=30, step=40, value=20)
      self.assertEqual(acc.FirstEventTimestamp(), 10)
  
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 18 for calling the MockingEventAccumulatorTest obj's underlying member method code with example usage: obj.testFirstEventTimestamp() without return types