[SPARK-21272] SortMergeJoin LeftAnti does not update numOutputRows

## What changes were proposed in this pull request?

Updating numOutputRows metric was missing from one return path of LeftAnti SortMergeJoin.

## How was this patch tested?

Non-zero output rows manually seen in metrics.

Author: Juliusz Sompolski <julek@databricks.com>

Closes #18494 from juliuszsompolski/SPARK-21272.

sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala

@@ -290,6 +290,7 @@ case class SortMergeJoinExec(
                 currentLeftRow = smjScanner.getStreamedRow
                 val currentRightMatches = smjScanner.getBufferedMatches
                 if (currentRightMatches == null || currentRightMatches.length == 0) {
+                  numOutputRows += 1
                   return true
                 }
                 var found = false

sql/core/src/test/scala/org/apache/spark/sql/execution/metric/SQLMetricsSuite.scala

@@ -483,6 +483,18 @@ class SQLMetricsSuite extends SparkFunSuite with SharedSQLContext {
     }
   }
 
+  test("SortMergeJoin(left-anti) metrics") {
+    val anti = testData2.filter("a > 2")
+    withTempView("antiData") {
+      anti.createOrReplaceTempView("antiData")
+      val df = spark.sql(
+        "SELECT * FROM testData2 ANTI JOIN antiData ON testData2.a = antiData.a")
+      testSparkPlanMetrics(df, 1, Map(
+        0L -> ("SortMergeJoin", Map("number of output rows" -> 4L)))
+      )
+    }
+  }
+
   test("save metrics") {
     withTempPath { file =>
       // person creates a temporary view. get the DF before listing previous execution IDs