Dedicated TermsBucket type to propagate sum_other_doc_count and doc_count_error_upper_bound

Author: gmarz

src/Nest/Aggregations/AggregationsHelper.cs

@@ -100,7 +100,19 @@ public DocCountBucket<SignificantTermItem> SignificantTerms(string key)
 				};
 		}
 
-		public Bucket<KeyedBucketItem> Terms(string key) => GetBucket<KeyedBucketItem>(key);
+		public TermsBucket Terms(string key)
+		{
+			var bucket = this.TryGet<BucketDto>(key);
+			return bucket == null
+				? null
+				: new TermsBucket
+				{
+					DocCountErrorUpperBound = bucket.DocCountErrorUpperBound,
+					SumOtherDocCount = bucket.SumOtherDocCount,
+					Items = bucket.Items.OfType<KeyedBucketItem>().ToList(),
+					Meta = bucket.Meta
+				};
+		}
 
 		public Bucket<HistogramItem> Histogram(string key)
 		{

src/Nest/Aggregations/Bucket/Terms/TermsBucket.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Nest
+{
+    public class TermsBucket : Bucket<KeyedBucketItem>
+    {
+		public long? DocCountErrorUpperBound { get; set; }
+		public long? SumOtherDocCount { get; set; }
+    }
+}

src/Tests/Aggregations/Bucket/Terms/TermsAggregationUsageTests.cs

@@ -94,6 +94,8 @@ protected override void ExpectResponse(ISearchResponse<Project> response)
 			response.IsValid.Should().BeTrue();
 			var states = response.Aggs.Terms("states");
 			states.Should().NotBeNull();
+			states.DocCountErrorUpperBound.Should().HaveValue();
+			states.SumOtherDocCount.Should().HaveValue();
 			foreach (var item in states.Items)
 			{
 				item.Key.Should().NotBeNullOrEmpty();