[SPARK-22135][MESOS] metrics in spark-dispatcher not being registered…

… properly

## What changes were proposed in this pull request?

Fix a trivial bug with how metrics are registered in the mesos dispatcher. Bug resulted in creating a new registry each time the metricRegistry() method was called.

## How was this patch tested?

Verified manually on local mesos setup

Author: Paul Mackles <pmackles@adobe.com>

Closes apache#19358 from pmackles/SPARK-22135.

resource-managers/mesos/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosClusterSchedulerSource.scala

@@ -23,8 +23,9 @@ import org.apache.spark.metrics.source.Source
 
 private[mesos] class MesosClusterSchedulerSource(scheduler: MesosClusterScheduler)
   extends Source {
-  override def sourceName: String = "mesos_cluster"
-  override def metricRegistry: MetricRegistry = new MetricRegistry()
+
+  override val sourceName: String = "mesos_cluster"
+  override val metricRegistry: MetricRegistry = new MetricRegistry()
 
   metricRegistry.register(MetricRegistry.name("waitingDrivers"), new Gauge[Int] {
     override def getValue: Int = scheduler.getQueuedDriversSize