Fix scala 2.13 compilation error

Author: ningyougang

core/invoker/src/main/scala/org/apache/openwhisk/core/containerpool/v2/FunctionPullingContainerPool.scala

@@ -19,7 +19,6 @@ package org.apache.openwhisk.core.containerpool.v2
 
 import java.util.concurrent.atomic.AtomicInteger
 import akka.actor.{Actor, ActorRef, ActorRefFactory, Cancellable, Props}
-
 import org.apache.openwhisk.common._
 import org.apache.openwhisk.core.connector.ContainerCreationError._
 import org.apache.openwhisk.core.connector.{
@@ -44,7 +43,7 @@ import org.apache.openwhisk.http.Messages
 
 import scala.annotation.tailrec
 import scala.collection.concurrent.TrieMap
-import scala.collection.immutable
+import scala.collection.{immutable, mutable}
 import scala.concurrent.Future
 import scala.concurrent.duration._
 import scala.util.{Random, Try}
@@ -116,10 +115,14 @@ class FunctionPullingContainerPool(
     val prewarmedSize = prewarmedPool.size
     val busySize = busyPool.size
     val warmedSize = warmedPool.size
-    val warmedPoolMap: Map[(String, String), Int] = warmedPool groupBy {
+    val warmedPoolGroupMap = warmedPool groupBy {
       case (_, warmedData) => (warmedData.invocationNamespace, warmedData.action.toString)
-    } mapValues (_.size)
-    for((data, size) <- warmedPoolMap) {
+    }
+    val warmedPoolResultMap: mutable.Map[(String, String), Int] = mutable.Map.empty[(String, String), Int]
+    warmedPoolGroupMap.foreach { data =>
+      warmedPoolResultMap += (data._1 -> data._2.size)
+    }
+    for ((data, size) <- warmedPoolResultMap) {
       val tags: Option[Map[String, String]] = Some(Map("namespace" -> data._1, "action" -> data._2))
       MetricEmitter.emitHistogramMetric(LoggingMarkers.INVOKER_CONTAINERPOOL_CONTAINER("warmed", tags), size)
     }