[SPARK-16439] [SQL] bring back the separator in SQL UI

## What changes were proposed in this pull request?

Currently, the SQL metrics looks like `number of rows: 111111111111`, it's very hard to read how large the number is. So a separator was added by #12425, but removed by #14142, because the separator is weird in some locales (for example, pl_PL), this PR will add that back, but always use "," as the separator, since the SQL UI are all in English.

## How was this patch tested?

Existing tests.
![metrics](https://cloud.githubusercontent.com/assets/40902/14573908/21ad2f00-030d-11e6-9e2c-c544f30039ea.png)

Author: Davies Liu <[email protected]>

Closes #15106 from davies/metric_sep.

Author: Davies Liu

sql/core/src/main/scala/org/apache/spark/sql/execution/metric/SQLMetrics.scala

@@ -18,6 +18,7 @@
 package org.apache.spark.sql.execution.metric
 
 import java.text.NumberFormat
+import java.util.Locale
 
 import org.apache.spark.SparkContext
 import org.apache.spark.scheduler.AccumulableInfo
@@ -101,8 +102,7 @@ object SQLMetrics {
    */
   def stringValue(metricsType: String, values: Seq[Long]): String = {
     if (metricsType == SUM_METRIC) {
-      val numberFormat = NumberFormat.getInstance()
-      numberFormat.setGroupingUsed(false)
+      val numberFormat = NumberFormat.getIntegerInstance(Locale.ENGLISH)
       numberFormat.format(values.sum)
     } else {
       val strFormat: Long => String = if (metricsType == SIZE_METRIC) {