expose num_reduce_phases as per elastic/elasticsearch#23288

Author: Mpdreamz

src/Nest/Search/Search/SearchResponse.cs

@@ -72,6 +72,11 @@ public interface ISearchResponse<T> : IResponse where T : class
 		/// </summary>
 		double MaxScore { get; }
 
+		/// <summary>
+		/// Number of times the server performed an incremental reduce phase
+		/// </summary>
+		long NumberOfReducePhases { get; }
+
 		/// <summary>
 		/// Gets the documents inside the hits, by deserializing <see cref="IHitMetadata{T}.Source"/> into T.
 		/// <para>NOTE: if you use <see cref="ISearchRequest.StoredFields"/> on the search request,
@@ -141,6 +146,9 @@ public class SearchResponse<T> : ResponseBase, ISearchResponse<T> where T : clas
 		[JsonProperty(PropertyName = "hits")]
 		public HitsMetaData<T> HitsMetaData { get; internal set; }
 
+		[JsonProperty(PropertyName = "num_reduce_phases")]
+		public long NumberOfReducePhases { get; internal set; }
+
 		[JsonIgnore]
 		public long Total => this.HitsMetaData?.Total ?? 0;