Fix #3328 Add periodic flush count to FlushStats (#3360)

This commit adds periodic flush count to FlushStats

src/Nest/CommonOptions/Stats/FlushStats.cs

@@ -9,8 +9,21 @@ public class FlushStats
 		[JsonProperty("total")]
 		public long Total { get; set; }
 
+		/// <summary>
+		/// The number of flushes that were periodically triggered when translog exceeded the flush threshold.
+		/// </summary>
+		[JsonProperty("periodic")]
+		public long Periodic { get; set; }
+
+		/// <summary>
+		/// The total time merges have been executed.
+		/// </summary>
 		[JsonProperty("total_time")]
 		public string TotalTime { get; set; }
+
+		/// <summary>
+		/// The total time merges have been executed (in milliseconds).
+		/// </summary>
 		[JsonProperty("total_time_in_millis")]
 		public long TotalTimeInMilliseconds { get; set; }