[SPARK-5762] Fix shuffle write time for sort-based shuffle

mateiz was excluding the time to write this final file from the shuffle write time intentional?

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #4559 from kayousterhout/SPARK-5762 and squashes the following commits:

5c6f3d9 [Kay Ousterhout] Use foreach
94e4237 [Kay Ousterhout] Removed open time metrics added inadvertently
ace156c [Kay Ousterhout] Moved metrics to finally block
d773276 [Kay Ousterhout] Use nano time
5a59906 [Kay Ousterhout] [SPARK-5762] Fix shuffle write time for sort-based shuffle

core/src/main/scala/org/apache/spark/util/collection/ExternalSorter.scala

@@ -723,6 +723,7 @@ private[spark] class ExternalSorter[K, V, C](
       partitionWriters.foreach(_.commitAndClose())
       var out: FileOutputStream = null
       var in: FileInputStream = null
+      val writeStartTime = System.nanoTime
       try {
         out = new FileOutputStream(outputFile, true)
         for (i <- 0 until numPartitions) {
@@ -739,6 +740,8 @@ private[spark] class ExternalSorter[K, V, C](
         if (in != null) {
           in.close()
         }
+        context.taskMetrics.shuffleWriteMetrics.foreach(
+          _.incShuffleWriteTime(System.nanoTime - writeStartTime))
       }
     } else {
       // Either we're not bypassing merge-sort or we have only in-memory data; get an iterator by