:: DeveloperApi ::
Information about an org.apache.spark.Accumulable modified during a task or stage.
Note: once this is JSON serialized the types of update and value will be lost and be
cast to strings. This is because the user can define an accumulator of any type and it will
be difficult to preserve the type in consumers of the event log. This does not apply to
internal accumulators that represent task level metrics.
partial value from a task, may be None if used on driver to describe a stage
total accumulated value so far, maybe None if used on executors to describe a task