Add support for date histogram format. (#3594)

(cherry picked from commit 0b8fb97)

Author: Stuart Cam

src/Nest/XPack/RollUp/RollupConfiguration/DateHistogramRollupGrouping.cs

@@ -12,29 +12,37 @@ namespace Nest
 	public interface IDateHistogramRollupGrouping
 	{
 		/// <summary>
-		/// How long to wait before rolling up new documents. By default, the indexer attempts to roll up all data that is available.
+		/// How long to wait before rolling up new documents. By default, the indexer attempts to roll up all data that is
+		/// available.
 		/// </summary>
-		[DataMember(Name ="delay")]
+		[DataMember(Name = "delay")]
 		Time Delay { get; set; }
 
 		/// <summary>
 		/// The date field that is to be rolled up
 		/// </summary>
-		[DataMember(Name ="field")]
+		[DataMember(Name = "field")]
 		Field Field { get; set; }
 
+		/// <summary>
+		/// Date format. Supports expressive date format pattern.
+		/// </summary>
+		[DataMember(Name = "format")]
+		string Format { get; set; }
+
 		/// <summary>
 		/// The interval of time buckets to be generated when rolling up. E.g. "60m" will produce 60 minute (hourly) rollups.
 		/// The interval defines the minimum interval that can be aggregated only.
 		/// </summary>
-		[DataMember(Name ="interval")]
+		[DataMember(Name = "interval")]
 		Time Interval { get; set; }
 
 		/// <summary>
-		/// Defines what time_zone the rollup documents are stored as. Unlike raw data, which can shift timezones on the fly, rolled
+		/// Defines what time_zone the rollup documents are stored as. Unlike raw data, which can shift timezones on the fly,
+		/// rolled
 		/// documents have to be stored with a specific timezone. By default, rollup documents are stored in UT
 		/// </summary>
-		[DataMember(Name ="time_zone")]
+		[DataMember(Name = "time_zone")]
 		string TimeZone { get; set; }
 	}
 
@@ -47,6 +55,9 @@ public class DateHistogramRollupGrouping : IDateHistogramRollupGrouping
 		/// <inheritdoc />
 		public Field Field { get; set; }
 
+		/// <inheritdoc />
+		public string Format { get; set; }
+
 		/// <inheritdoc />
 		public Time Interval { get; set; }
 
@@ -61,6 +72,8 @@ public class DateHistogramRollupGroupingDescriptor<T>
 	{
 		Time IDateHistogramRollupGrouping.Delay { get; set; }
 		Field IDateHistogramRollupGrouping.Field { get; set; }
+
+		string IDateHistogramRollupGrouping.Format { get; set; }
 		Time IDateHistogramRollupGrouping.Interval { get; set; }
 		string IDateHistogramRollupGrouping.TimeZone { get; set; }
 
@@ -78,5 +91,8 @@ public class DateHistogramRollupGroupingDescriptor<T>
 
 		/// <inheritdoc cref="IDateHistogramRollupGrouping.TimeZone" />
 		public DateHistogramRollupGroupingDescriptor<T> TimeZone(string timeZone) => Assign(a => a.TimeZone = timeZone);
+
+		/// <inheritdoc cref="IDateHistogramRollupGrouping.Format" />
+		public DateHistogramRollupGroupingDescriptor<T> Format(string format) => Assign(a => a.Format = format);
 	}
 }