[SPARK-13052] waitingApps metric doesn't show the number of apps currently in the WAITING state

Author: Raafat Akkad <raafat.akkad@gmail.com>

Closes #10959 from RaafatAkkad/master.

core/src/main/scala/org/apache/spark/deploy/master/Master.scala

@@ -74,7 +74,7 @@ private[deploy] class Master(
 
   val workers = new HashSet[WorkerInfo]
   val idToApp = new HashMap[String, ApplicationInfo]
-  val waitingApps = new ArrayBuffer[ApplicationInfo]
+  private val waitingApps = new ArrayBuffer[ApplicationInfo]
   val apps = new HashSet[ApplicationInfo]
 
   private val idToWorker = new HashMap[String, WorkerInfo]

core/src/main/scala/org/apache/spark/deploy/master/MasterSource.scala

@@ -42,6 +42,6 @@ private[spark] class MasterSource(val master: Master) extends Source {
 
   // Gauge for waiting application numbers in cluster
   metricRegistry.register(MetricRegistry.name("waitingApps"), new Gauge[Int] {
-    override def getValue: Int = master.waitingApps.size
+    override def getValue: Int = master.apps.filter(_.state == ApplicationState.WAITING).size
   })
 }