Gate heartbeat success logs (#8407)

src/main/scala/net/eagle0/eagle/service/EagleServiceImpl.scala

@@ -425,9 +425,11 @@ class EagleServiceImpl(
       // causing clients to see a sync mismatch before receiving all updates.
       // This particularly affects slower/remote connections (e.g., Windows clients).
       gamesManager.synchronized {
-        println(
-          s"got heartbeat from $userId. Client timestamp ${request.clientTimestamp}, server timestamp ${System.currentTimeMillis()}"
-        )
+        val serverTimestamp = System.currentTimeMillis()
+        if EagleServiceImpl.LogHeartbeats then
+          SimpleTimedLogger.printLogger.logLine(
+            s"got heartbeat from $userId. Client timestamp ${request.clientTimestamp}, server timestamp $serverTimestamp"
+          )
 
         // Verify sync status for each game the client reports
         val gameSyncResults = request.gameSyncStatuses.flatMap { clientGameStatus =>
@@ -492,7 +494,7 @@ class EagleServiceImpl(
           UpdateStreamResponse(
             responseDetails = UpdateStreamResponse.ResponseDetails.HeartbeatResponse(
               HeartbeatResponse(
-                serverTimestamp = System.currentTimeMillis(),
+                serverTimestamp = serverTimestamp,
                 gameSyncResults = mismatchedResults
               )
             )
@@ -687,6 +689,12 @@ class EagleServiceImpl(
 }
 
 object EagleServiceImpl {
+  private val EnabledEnvValues = Set("1", "true", "yes", "on")
+  private val LogHeartbeats    =
+    sys.env
+      .get("EAGLE_LOG_HEARTBEATS")
+      .exists(value => EnabledEnvValues.contains(value.trim.toLowerCase))
+
   private[service] def tutorialStartPhaseForCreateGame(request: CreateGameRequest): Option[TutorialPhase] =
     if request.tutorialStartPhase != TutorialPhaseProto.TUTORIAL_PHASE_UNKNOWN then
       Some(TutorialPhaseConverter.fromProto(request.tutorialStartPhase))