how to calculate number of markers inside a polygo

Looking for a way to count the number of markers inside a polygon which is drawn dynamically by the user

The code below can draw the polygon now i want to show the count of number of markers inside the polygon if there exists

<!DOCTYPE html>
<html>
<head>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no">
    <meta charset="utf-8">
    <title>Simple Polygon</title>
    <style>
        html, body {
            height: 100%;
            margin: 0;
            padding: 0;
        }
        #map {
            height: 100%;
            width: 100%;
        }
    </style>
</head>
<body>
<div id="map"></div>
<script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?v=3.17&AIzaSyBLnIMoOLvgLaeIUEzLX3sIw0-FOXwXcag"> </script>
<script>
    function initMap() {
        var mapOptions={
            center:{lat: 27.174977,lng:78.042064},
            zoom: 8
        };
        var map15= new google.maps.Map(document.getElementById("map"),mapOptions);

        var polygon= new google.maps.Polygon({
            strokeColor: "#1E41AA",
            strokeOpacity: 1.0,
            strokeWeight: 3,
            map: map15,
            fillColor: "#2652F2",
            fillOpacity: 0.6
        });

        var poly= polygon.getPath();
        function addPolyPoints(e){
            poly.push(e.latLng);
        }

        google.maps.event.addListener(map15,'click',addPolyPoints);
    }
    initMap();
</script>
</body>
</html>

ie, when the user draw the polygon the map if there exists a marker the count should be provided.

How to do it?

keep references to your markers in an array
iterate through that array using google.maps.geometry.poly.containsLocation(latLng,polygon) to determine if the marker is in the polygon or not
output the result.

proof of concept fiddle

code snippet:

function initMap() {
  var mapOptions = {
    center: {
      lat: 27.174977,
      lng: 78.042064
    },
    zoom: 8
  };
  var map15 = new google.maps.Map(document.getElementById("map"), mapOptions);
  var markers = []
  google.maps.event.addListenerOnce(map15, 'bounds_changed', function() {
    function getRandom(min, max) {
      return Math.random() * (max - min + 1) + min;
    }
    var bounds = map15.getBounds();
    for (var j = 0; j < 30; j++) {

      var marker = new google.maps.Marker({
        position: new google.maps.LatLng(getRandom(bounds.getSouthWest().lat(), bounds.getNorthEast().lat()),
          getRandom(bounds.getSouthWest().lng(), bounds.getNorthEast().lng())),
        map: map15
      });
      markers.push(marker);
    }

  })

  var polygon = new google.maps.Polygon({
    strokeColor: "#1E41AA",
    strokeOpacity: 1.0,
    strokeWeight: 3,
    map: map15,
    fillColor: "#2652F2",
    fillOpacity: 0.6
  });

  var poly = polygon.getPath();

  function addPolyPoints(e) {
    poly.push(e.latLng);
    var markerCnt = 0;
    for (var i = 0; i < markers.length; i++) {
      if (google.maps.geometry.poly.containsLocation(markers[i].getPosition(), polygon)) {
        markerCnt++;
      }
    }
    document.getElementById('info').innerHTML = "markers in polygon: " + markerCnt;
  }

  google.maps.event.addListener(map15, 'click', addPolyPoints);
}

google.maps.event.addDomListener(window, "load", initMap);

html,
body,
#map {
  height: 100%;
  width: 100%;
  margin: 0px;
  padding: 0px
}

<script src="https://maps.googleapis.com/maps/api/js?libraries=geometry&key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk"></script>
<div id="info"></div>
<div id="map"></div>