Huge number of OpenRequest in spray-can statistics

Question

I'm using spray-can version 1.3.3 and akka 2.3.9 The server code looks pretty standard

application.conf:

spray.can {
 server {
  pipelining-limit = 10
  request-timeout = 50ms
  stats-support = on
 }
...

Boot:

object Boot extends App {
  implicit val system = ActorSystem("on-spray-can")
  val service = system.actorOf(Props[MyServiceActor], "demo-service")
  IO(Http) ! Http.Bind(service, "0.0.0.0", port = 8080)
 }

MySeviceActor:

  class MyServiceActor extends Actor with MyService {
    def actorRefFactory = context
    def receive = runRoute(route)
  }

trait MyService extends HttpService {
  implicit def executionContext = actorRefFactory.dispatcher
 val route: Route =
   path("do") {
     post {
       detach() {          
          entity(as[String]) { body =>
            complete {
              val resp = processRequestAsync(body)
              convertToHttpResponseAsync(resp)
            }}}}
 }

 def processRequestAsync(body:String):Future[MyContext]={
     // do some non-blocking using async client.api, spray-client..
 }
 def convertToHttpResponseAsync(resp: Future[MyContext]): Future[HttpResponse] = {
  resp.map { ctx => 
 // ... Do some non blocking ops
    HttpResponse(StatusCodes.OK, entity = HttpEntity(`application/json`, ctx.getData))
  }
 }
}

In my localhost environment the statistics looks reasonable (at least #of open connections) I'm using ab, wrk benchmarks, But in production I'm getting very different picture:

Total requests        : 2434475
Open requests         : 1847228
Max open requests     : 1847228
Total connections     : 1995540
Open connections      : 142
Max open connections  : 1239
Requests timed out    : 189196

There is an ngnix that receives the outside requests and dispaches them to the application.

The question is what could be a root cause for so many open connections? What parameters, headers, something else should I pay attention, investigate?

My application runs from docker container