[SPARK-16827][BRANCH-2.0] Avoid reporting spill metrics as shuffle metrics

## What changes were proposed in this pull request?

Fix a bug where spill metrics were being reported as shuffle metrics. Eventually these spill metrics should be reported (SPARK-3577), but separate from shuffle metrics. The fix itself basically reverts the line to what it was in 1.6.

## How was this patch tested?

Cherry-picked from master (#15347)

Author: Brian Cho <bcho@fb.com>

Closes #15455 from dafrista/shuffle-metrics-2.0.

Author: bchocho

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeExternalSorter.java

@@ -144,7 +144,9 @@ private UnsafeExternalSorter(
     // Use getSizeAsKb (not bytes) to maintain backwards compatibility for units
     // this.fileBufferSizeBytes = (int) conf.getSizeAsKb("spark.shuffle.file.buffer", "32k") * 1024;
     this.fileBufferSizeBytes = 32 * 1024;
-    this.writeMetrics = taskContext.taskMetrics().shuffleWriteMetrics();
+    // The spill metrics are stored in a new ShuffleWriteMetrics, and then discarded (this fixes SPARK-16827).
+    // TODO: Instead, separate spill metrics should be stored and reported (tracked in SPARK-3577).
+    this.writeMetrics = new ShuffleWriteMetrics();
 
     if (existingInMemorySorter == null) {
       this.inMemSorter = new UnsafeInMemorySorter(