SPARK-2900. aggregate inputBytes per stage

This is slightly hard to read. Can you write it this way
```
val inputBytesDelta = {
  val newBytes = taskMetrics.inputMetrics.map(_.bytesRead).getOrElse(0L)
  val oldBytes = oldMetrics.flatMap(_.inputMetrics).map(_.bytesRead).getOrElse(0L)
  newBytes - oldBytes
}
```