[SPARK-20136][SQL] Add num files and metadata operation timing to scan operator metrics

## What changes were proposed in this pull request?
This patch adds explicit metadata operation timing and number of files in data source metrics. Those would be useful to include for performance profiling.

Screenshot of a UI with this change (num files and metadata time are new metrics):

<img width="321" alt="screen shot 2017-03-29 at 12 29 28 am" src="https://cloud.githubusercontent.com/assets/323388/24443272/d4ea58c0-1416-11e7-8940-ecb69375554a.png">

## How was this patch tested?
N/A

Author: Reynold Xin <rxin@databricks.com>

Closes #17465 from rxin/SPARK-20136.

sql/core/src/main/scala/org/apache/spark/sql/execution/DataSourceScanExec.scala

@@ -171,8 +171,20 @@ case class FileSourceScanExec(
     false
   }
 
-  @transient private lazy val selectedPartitions =
-    relation.location.listFiles(partitionFilters, dataFilters)
+  @transient private lazy val selectedPartitions: Seq[PartitionDirectory] = {
+    val startTime = System.nanoTime()
+    val ret = relation.location.listFiles(partitionFilters, dataFilters)
+    val timeTaken = (System.nanoTime() - startTime) / 1000 / 1000
+
+    metrics("numFiles").add(ret.map(_.files.size.toLong).sum)
+    metrics("metadataTime").add(timeTaken)
+
+    val executionId = sparkContext.getLocalProperty(SQLExecution.EXECUTION_ID_KEY)
+    SQLMetrics.postDriverMetricUpdates(sparkContext, executionId,
+      metrics("numFiles") :: metrics("metadataTime") :: Nil)
+
+    ret
+  }
 
   override val (outputPartitioning, outputOrdering): (Partitioning, Seq[SortOrder]) = {
     val bucketSpec = if (relation.sparkSession.sessionState.conf.bucketingEnabled) {
@@ -293,6 +305,8 @@ case class FileSourceScanExec(
 
   override lazy val metrics =
     Map("numOutputRows" -> SQLMetrics.createMetric(sparkContext, "number of output rows"),
+      "numFiles" -> SQLMetrics.createMetric(sparkContext, "number of files"),
+      "metadataTime" -> SQLMetrics.createMetric(sparkContext, "metadata time (ms)"),
       "scanTime" -> SQLMetrics.createTimingMetric(sparkContext, "scan time"))
 
   protected override def doExecute(): RDD[InternalRow] = {