Fixes #4948 | Lazy event parsing in HTTP callbacks. (#5114)

We can use `jsonString` form `MarathonEvent` to ommit double parsing
evnets.

src/main/scala/mesosphere/marathon/core/event/impl/callback/HttpEventActor.scala

@@ -2,16 +2,13 @@ package mesosphere.marathon.core.event.impl.callback
 
 import akka.actor._
 import akka.pattern.ask
-import mesosphere.marathon.api.v2.json.Formats._
 import mesosphere.marathon.core.base.Clock
-import mesosphere.marathon.core.event.impl.callback.HttpEventActor._
 import mesosphere.marathon.core.event._
+import mesosphere.marathon.core.event.impl.callback.HttpEventActor._
 import mesosphere.marathon.core.event.impl.callback.SubscribersKeeperActor.GetSubscribers
 import mesosphere.marathon.metrics.{ MetricPrefixes, Metrics }
-import play.api.libs.json.JsValue
 import spray.client.pipelining.{ sendReceive, _ }
 import spray.http.{ HttpRequest, HttpResponse }
-import spray.httpx.PlayJsonSupport
 
 import scala.concurrent.duration._
 import scala.concurrent.{ ExecutionContext, Future }
@@ -60,7 +57,7 @@ class HttpEventActor(
   subscribersKeeper: ActorRef,
   metrics: HttpEventActorMetrics,
   clock: Clock)
-    extends Actor with ActorLogging with PlayJsonSupport {
+    extends Actor with ActorLogging {
 
   implicit val timeout = conf.eventRequestTimeout
   def pipeline(implicit ec: ExecutionContext): HttpRequest => Future[HttpResponse] = {
@@ -96,8 +93,7 @@ class HttpEventActor(
     //remove all unsubscribed callback listener
     limiter = limiter.filterKeys(subscribers.urls).iterator.toMap.withDefaultValue(NoLimit)
     metrics.skippedCallbacks.mark(limited.size)
-    val jsonEvent = eventToJson(event)
-    active.foreach(url => Try(post(url, jsonEvent, self)) match {
+    active.foreach(url => Try(post(url, event, self)) match {
       case Success(res) =>
       case Failure(ex) =>
         log.warning(s"Failed to post $event to $url because ${ex.getClass.getSimpleName}: ${ex.getMessage}")
@@ -106,18 +102,18 @@ class HttpEventActor(
     })
   }
 
-  def post(url: String, event: JsValue, eventActor: ActorRef): Unit = {
+  def post(url: String, event: MarathonEvent, eventActor: ActorRef): Unit = {
     log.info("Sending POST to:" + url)
 
     metrics.outstandingCallbacks.inc()
     val start = clock.now()
-    val request = Post(url, event)
+    val request = Post(url, event.jsonString)
 
     val response = pipeline(context.dispatcher)(request)
 
     import context.dispatcher
     response.onComplete {
-      case _ =>
+      _ =>
         metrics.outstandingCallbacks.dec()
         metrics.callbackResponseTime.update(start.until(clock.now()))
     }