finagle-stats: Changed CumulativeGauge to use FuturePool.defaultExecutor

Problem

Finagle uses FJPool.commonPool for managing its gauges, which adds an extra
thread pool to worry about.

Solution

It's unlikely that gauge management will be a huge burden, so let's just use
FuturePool#defaultExecutor and call it a day.

JIRA Issues: CSL-8357

Differential Revision: https://phabricator.twitter.biz/D455975

finagle-stats-core/src/main/scala/com/twitter/finagle/stats/MetricsStatsReceiver.scala

@@ -6,6 +6,7 @@ import com.twitter.finagle.util.DefaultTimer
 import com.twitter.logging.{Level, Logger}
 import com.twitter.util.{Future, FuturePool, Time}
 import com.twitter.util.lint.{Category, GlobalRules, Issue, Rule}
+import java.util.concurrent.Executor
 import java.util.concurrent.atomic.LongAdder
 import scala.collection.JavaConverters._
 
@@ -53,6 +54,12 @@ class MetricsStatsReceiver(val registry: Metrics)
     with CollisionTrackingStatsReceiver
     with WithHistogramDetails {
 
+  /**
+   * Overrides the executor that manages cumulative gauges to use the same
+   * executor that backs FuturePool.unboundedPool.
+   */
+  override def executor: Executor = FuturePool.defaultExecutor
+
   import MetricsStatsReceiver._
 
   def this() = this(MetricsStatsReceiver.defaultRegistry)