Log changes to the ShardCoordinator ddata state (#27444)

The number of shards is configurable, in the order of magnitude of the number
of nodes in the cluster. Logging the ActorRef for each allocated shard is
useful to see on which node the shard is allocated.

akka-cluster-sharding/src/main/scala/akka/cluster/sharding/ShardCoordinator.scala

@@ -329,6 +329,8 @@ object ShardCoordinator {
         rememberEntities: Boolean = false)
         extends ClusterShardingSerializable {
 
+      override def toString = s"State($shards)"
+
       def withRememberEntities(enabled: Boolean): State = {
         if (enabled)
           copy(rememberEntities = enabled)
@@ -1049,6 +1051,7 @@ class DDataShardCoordinator(
     ({
       case g @ GetSuccess(CoordinatorStateKey, _) =>
         state = g.get(CoordinatorStateKey).value.withRememberEntities(settings.rememberEntities)
+        log.debug("Received initial coordinator state [{}]", state)
         val newRemainingKeys = remainingKeys - CoordinatorStateKey
         if (newRemainingKeys.isEmpty)
           becomeWaitingForStateInitialized()
@@ -1178,6 +1181,7 @@ class DDataShardCoordinator(
   private def unbecomeAfterUpdate[E <: DomainEvent](evt: E, afterUpdateCallback: E => Unit): Unit = {
     context.unbecome()
     afterUpdateCallback(evt)
+    log.debug("New coordinator state after [{}]: [{}]", evt, state)
     unstashGetShardHomeRequests()
     unstashAll()
   }
@@ -1236,6 +1240,7 @@ class DDataShardCoordinator(
 
   def sendCoordinatorStateUpdate(evt: DomainEvent) = {
     val s = state.updated(evt)
+    log.debug("Publishing new coordinator state [{}]", state)
     replicator ! Update(CoordinatorStateKey, LWWRegister(selfUniqueAddress, initEmptyState), writeMajority, Some(evt)) {
       reg =>
         reg.withValueOf(s)