add numSteps property and update recorder script

com.unity.ml-agents/Runtime/Demonstrations/DemonstrationRecorder.cs

         public bool Record;
 
         /// <summary>
+        /// Number of steps to record. Editor will stop playing when it hits this threshold.
+        /// </summary>
+        public int numStepsToRecord = 5000;
+
+        /// <summary>
         /// Base demonstration file name. If multiple files are saved, the additional filenames
         /// will have a sequence of unique numbers appended.
         /// </summary>
             if (Record)
             {
                 LazyInitialize();
+            }
+
+            if (m_DemoWriter.NumSteps >= numStepsToRecord)
+            {
+                UnityEditor.EditorApplication.isPlaying = false;
             }
         }
 

com.unity.ml-agents/Runtime/Demonstrations/DemonstrationWriter.cs

         }
 
         /// <summary>
+        /// Number of steps written so far.
+        /// </summary>
+        internal int NumSteps
+        {
+            get { return m_MetaData.numberSteps; }
+        }
+
+        /// <summary>
         /// Writes the initial data to the stream.
         /// </summary>
         /// <param name="demonstrationName">Base name of the demonstration file(s).</param>